Active Worlds:
You are allowed to be a "Tourist" in AW for as long as you want, no credit card required (I think). Being a Tourist does limit your abilities to some extent, only a few worlds are open to Tourists, there are few of these that allow Tourists to build, your avatar choices in each world will be limited, and other features may be disabled to you. Your text will also appear grey/gray as a Tourist and you will have quotes around your name as well. Download the software and login as a Tourist, then sign up whenever you wish.

Second Life:
A free Seven(7) Day trial is available, credit card is required (for proof of parental approval, there is a teen grid but A CC is still required). Only one free trail per credit card in each 90-day period, meaning you can not have multiple trial accounts at the same time (this is to limit abuse). There are very few if any restrictions on trial users. You recieve 250L$ as a starter sum for basic account type and 500L$ for premium, with the possibility to make more of course. I believe at the end of the 7-days you have to cancel your account by signing in on the website and going to account management (secondlife.com - Login - Your Account (upper right corner) - Cancel Account (right side)) or else you will be charged.

There:
A free Three(3) Day trial is offered, no credit card required. I'm not sure what if any restrictions are applied to trial users. To actualy register I believe a credit card is indeed needed (duh). I think you do not get use of things such as the Compass, Voice Chat, Some Teleportation is restricted, and some other purchasing limitations(?), maybe.

Active Worlds:
Only available for Windows (98 to XP I think). Their website reports it can be run under emulation on Mac OS, and probably on Linux as well (under a windows emulator perhaps). No other native clients available.

Second Life:
Currently a Windows and Mac OS X client are available with other Mac clients in testing I believe. A Linux client is in the works as well, not sure when it will be released, sometime within 2005. Some have been able to successfully run SL under emulation on Linux, recently I think this has become harder to do, but still may be possible.

There:
There is only available for Windows (98 SE to XP). I have not heard of anyone running There under emulation on Mac or Linux or even attempting. This may have to do with There's strong integration of IE or its rendering (no idea). A list of supported OS can be found Here (note There's website wants you to use IE and will give you a BS message if you're using anything else).

Active Worlds:
AW features 3D graphics using the Renderware engine (same used by GTA and other various games). Many different visual effects are possible, such as some basic particles, light sources, and other basic stuff. While AW does use Renderware, it does not look anything like GTA, alot of the default models and graphics are "dated" looking. There are some great worlds that look very nice and neat with custom models (and some custom avatars ) but these are still somewhat restricted by AW's use of Renderware. This does allow for lower end computers to use AW without much damage however. Several different choices are available in the preferences for how you want to render the graphics (OpenGL, Software, and Hardware Accelerated I believe).

Second Life:
SL uses a home-brewed graphics engine I believe. Avatar base models (which are user customizable) are created in Maya I think. Pretty customizable particle creation, real time shadowing engine, and other basic 3D stuff. Rendering is done through OpenGL. Work is currently being done on the graphics engine for improvements and such.

There:
There also uses a home-made engine. Avatars and general visual style is that of "cartoony" expresionalism and caricatureism. Avatars are made so that facial expressions and body movements are braught more to life. User created models are limited to a certain number of polygons. LOD (Level Of Detail) models are used as well. I think defaultly There uses DirectX as a render engine, but recently has implemented the ability for you to optionaly use OpenGL (I could be wrong on this).

Active Worlds:
Support varies in a few forms in AW. There is a Support Form as well as the usual FAQ and documentation. There is also two groups of user volunteers. One is the GateKeepers mostly seen in worlds such as AWGate, they act as administrators as well as answering questions (not all GK's are good at technical questions). The other group is the PeaceKeepers, a group found in a few different worlds such as AlphaWorld, they act in simular in style to GKs but aren't restricted to AWGate, they can be found in most AW and some privately owned worlds.

Second Life:
Second Life has multiple ways of getting support. One way is by simply emailing support@secondlife.com. Another way is by IMing a Liaison in world, a Liaison is a Linden Lab employee who helps users in-world, however not all of them know the answer to everything of course. Yet another way is by using something called "Live Help" which is group of residents who dedicate their time to take questions (simular to AW's GKs and PKs), one can ask a question by going to Help - Live Help. SL also has a Support Wiki (login required) and Forums (login required) with a Technical Issues section where you can ask questions.

There:
There support goes mainly through something called "There Care", which is a Knowledge Base setup, allowing you to search for your question. You can also email for support help@there.com, or post a question on the forums (I think there is a support or technical issues category). There used to allow you to IM helpers about questions or for support, but I think that service was discontinued, but may pick up again at some time.

Active Worlds:
There is no "global" economy, meaning there is no economy that runs through-out all worlds. However, some worlds do have an economy within them, and controlled by bots. I'm not sure how controlled these economies are. So AW does and doesn't have an economy.

Second Life:
SL's economy is the LindenDollar(often shortened to "L$"). It is a well controlled and closely watched economy system(monitored by Linden Lab). It seems every aspect of the economy is watched, as spreadsheets are released regularly in which statistics and trends about the economy are found. Changes to the economy are also made to avoid situations like inflation, devaluation, and etc. L$ is also traded on GOM (GamingOpenMarket.com) and sold on IGE (IGE.com) for USD(US Currency).

There:
There's economy is ThereBucks (often shortened to TBux). TBux are persistant through-out the world. Some base prices (submission fees for developers, developer created items, etc.) are dropped or raised (mostly dropped) every now and then. I'm not sure why this is, if it is to avoid real economic issues , or if it is just because of user demand. I honestly do not know how tightly controled There's economy is. Generaly speaking however, alot of users find items to be "overpriced", and find there to be "ALOT" of things that cost money. TBux are also traded and sold for USD, there are a few different sites that they can be bought from, There.com itself, as well as some other user run (and pretty well trusted) established sites.

Active Worlds:
Has no real physics. The only feeling of physics is from avatars, they can jump, fall, and swim. Physics could be emulated through a bot perhaps, but I am not aware of this being done.

Second Life:
Fully featured physics running on the Havok 1 (soon Havok 2) engine. Supports basic object physics (primitive shapes set physical will fall to the ground and become pushable) and avatar interaction physics (avatars are solid and collisional). Limited friction is possible and objects do not float in water automaticly (but can be scripted to do so). No rag-doll on avatars.

There:
Uses in-house built physics engine (I believe), fully featured. Also has rag-doll effect on avatars. Vehicle physics are agreed by most to be "the best" out of current virtual worlds, even most SL users agree with this. Vehicles feature such things as suspension, some wind reactions, and some friction reactions.

Active Worlds:
You can only use content created by yourself if you own a world. Avatars and objects are transfered to clients from world Object Paths (basicly content servers), these object paths are usualy only available to world owners. In order to have custom content (avatar or 3D-Model) you have to upload the models to your OP. However, some other things such as textures on certain objects are available to anyone if the world permits its use, some objects can display textures from URLs, as well as music, and video streams. Model file type can be either RWX or COB I believe, Truespace is usualy a recommended program to use (although any 3D modeling program that exports to those two are fine I'm sure).

Second Life:
Since most content in SL is made with in-world tools anyone can create most types of content at any time or place (depending on the land you're on and its settings). This includes, but is not limited to, complex objects made using primitive shapes, avatars, textures, sounds, animations, etc.. Some content such as textures, sounds, and animations may require 3rd party software to make (such as photoshop, a sound editing program, and poser or some other 3D animation software) and then upload to SL. Uploading content costs 10L$ for each item uploaded. An in-world editor for creating animations is said to be in the works, but I do not know the status or a time-table of this. Clothing and avatar skins are made from basic textures (templates are available to help with this). Animations must be of BHV file type, and some settings have to be applied. Images can be JPG, BMP, or TGA. Sounds are restricted to WAV and must be shorter than 10 seconds I think.

There:
Anyone is free to make most forms of content, fees and submission processing is needed for almost every type of content however. Users can make basic objects, animated objects, some vehicles (not all types), clothing, some houses, some furniture, etc.. Users can not make complex scripted items, models over the set polygon limits, some vehicles, pets, avatar models, avatar model items (hair, hats, etc), and other random stuff. Fees vary depending on what type of item you want to submit, they can range from expensive to REALLY expensive, there are submission fees as well as fees you pay on each sale made, there are programs to help first-time sellers to overcome these fees. A submission process is needed as well, an employee of There checks your product to see if it goes against any rules (copyright, rating, polylimits), this can take anywhere from a few days to a week and often whether your product makes it or not depends on who is approving it (some are more forgiving than others). There works alot like a catalog in its item handling, content is listed in a catalog like shop website where users can browse, you can set/buy how many products you want to sell, working alot like as if you are paying for materials to make the item (really its for webspace). You can then use the content wherever you wish (unless the current zone doesn't allow you to, i.e. no-drop zones). Clothing is gender based (males can not wear female shirts and vice-versa). Alot of itmes are made using pre-made models which users basicly retexture with custom textures and apply in a There created previewer product. Models are required to be made in GMax (by Discreet).

Active Worlds:
Custom avatars are only available if you own a world. Since avatars are stored on each worlds Object Path you can only use the avatars made available to you in that world. You can not make a custom avatar and use it in all worlds. This is a feature long wanted by many AW users. Avatar files are .rwx (RenderWare) files (zipped), the textures used on avatars are .jpg, and then the .rwx file is listed in an avatars.dat file (zipped) which also defines the custom animations (thank you to Velox for this information).

Second Life:
Avatars can be customized greatly using in-world sliders. Everything from your nose to your hips can be customized in how they look. Skins and other textures can be easily used and applied to upper body, lower boddy, face, hair, and eyes. You can freely make and save as many avatars as you wish with no charge (not inclueding any upload fees for textures). Custom avatars are also sold by many vendors, as well as things to go with avatars such as hair, skin , shapes, and other various items. Objects/prims can be attached to avatars and worn, allowing for users to make other things not possible with sliders alone, such as animal shapes, robots, and anything else you want your avatar to be. Custom avatars can be worn anywhere, the only restrictions being on the rating of the sim (R or PG) and if your avatar complies to that rating.

There:
Users can freely shape avatars at Spas located in different areas of There. Avatars are restricted to humanoid looking and unnatural skintones can only be applied through clothing. You can shape your avatar from thin to over-weight with the sliders at the Spa, anything from head shape to stomach size. Things such as hair color, eye color, skin tone, and such are allowed to a degree in the spa. Some things like weird colors or shapes you can only buy from the shop. You can only save one free avatar at a time, you can make more than one avatar shape/color set, but must pay extra for each additional avatar. Items can not be attached to your avatar, but accessories such as hats (with predefined hair colors/styles), glasses, lipstick, eye "contacts", shoes, pants, shirts, and more, are all available through the shop. Avatars are not restricted to any one area, you can freely use the same custom avatar everywhere, some areas do not allow you to change clothes/shapes/avatar stuff though.

Active Worlds:
Animations like most things in AW are stored on each objects local Object Path. You can specify which avatars can use which animations. I'm not sure exactly how you make animations, I believe Truespace can be used in someway.

Second Life:
Any user can make and use a custom animation. Animations are made in any 3D Animation program , Poser is suggested however, it may be difficult to use other programs, BHV is the file type used. Anyone is free to create, upload, and distribute (give or sell) animations. Some bones are not allowed to be animated (Poser 2 model is suggested) such as the face and individual fingers/toes. Facial expressions and finger gestures can be selected upon uploading the animation. Things such as looping and some other options can be set at uploading as well. Custom animations are used alot around Second Life, present in things such as customizing your walk, to furniture, to vehicles, and other random scripts. Animations can be called from scripts, put into "Gestures" (macro like things), or used manualy from the Inventory.

There:
Currently there is no way to create custom animations in There. Animations are made by There developers only and new animations are released every now-and-then in updates. Alot of users have taken to stringing together (either manualy or through use of popular macro programs) animations to make custom like animation chains. To There developers credit, the animations the avatars in There have are widely agreed to be some of the most life-like and smoothest of the virtual worlds, the facial expressions especialy.

Active Worlds:
Allows for building using predefined object models. These models are determined by each worlds "Object Path"(OP), which is basicly a folder on a server in which the files of all the objects you can use are kept. Most worlds that support building use the "first come first serv" model, where you can only build on bare land and ontop of your own objects, or ontop of other peoples objects if you have their "Privileges". Putting down an object is basicly claiming your own space and is free.

Second Life:
You can use basic "primitive"(prim(s)) shapes to build more complex "objects" (a group of prims). Each shape can be altered in different ways to make more unique shapes. You can not use any pre-made 3D models. Building is allowed on land you own, or in public "sandboxes" where anyone is free to build. You can also build on other peoples lands if they allow it, its proper to ask before doing so wildly of course.

There:
Building is done with pre-uploaded 3D models made by users or There developers. A user may build in a Port-A-Zone(PAZ) or in a house in which they have permission to drop. You can not "staticly" or "stream" build within There (i.e. with building blocks) and can not really make new shapes unless you have a set of 3D models that allow you to do so (there are some user made models that look like building "sets"). You can *place* objects down however in real time, i.e. I can put down a chair in my PAZ and everyone will see me do it, but I can't build a chair infront of everyone out of a shape. Models are required to be made with GMax (by Discreet).

Active Worlds:
Some basic scripting is available. Each object can contain some code, the code is based on a AW created "language", which works more like command line arguements given to an object. It is not like a typical scripting language. It can be made very usefull for basic things however, such as particle effects, doors that open/close, moving objects, setting textures via url, setting text on an object (specific objects that allow it), teleporting users when they click an object, etc. The "scripting" basicly allows you to do object operations using commands predefined by AW. There is also an SDK made available which allows you to create "bots" to do much more complex activities, everything from playing kickball to random bird generators to just about anything a normal avatar could do in AW.

Second Life:
SL has its own "full-featured" scripting language (LSL, Linden Script(ing) Language). LSL is very simular to C++ and Java in its syntax and setup. Basic scripting operations are present, such as variables, math operations, boolean expressions, and pretty much everything else. LSL is not Object Oriented in the true sense of the phrase. LSL allows you to manipulate objects and other things within SL. LSL can be very powerfull, but does contain some annoying limitations, such as memory size (limited to 16k mb(?)), and lacks some features that could help. Some limitations can be bypassed however using XML-RPC and emailing, both which LSL supports. LSL is constantly being developed, improved, and bug fixed. It is used by a very wide range of users to do a great veriaty of things, from AI to basic calculators, there is even a Wiki dedicated strictly to LSL.

There:
Currently There has no in-world scripting available to users. Supposedly there was once plans to allow users to create their own scripted objects, but that plan apparently fell through or is on indefinit hold. Since a large portion of There is Flash based (mostly GUI) some users have taken to "hacking" There files as well as making their own additions and programs. Also some users have created various "patches" and "add-ons" (non-Flash based). I don't believe There has ever given an official comment on these practices, but they have not stopped it either, mostly because the hacks/patches do not harm anyone and add better functionality to There making it more enjoyable. Users can also create some basic interactive objects (models with walk-through walls and animated parts), but this is through the 3D modeler GMax which is required to make items with, not through any scripting (I think).

Active Worlds:
Voice chat is not supported in AW. Video streaming is supported with audio however, so potentialy you could setup to live streams and voice chat that way, not sure how easy that would be to setup however.

Second Life:
SL does not have voice chat. SL does however allow for users to setup audio-streams from their land parcels (only avatars on the land can hear the audio stream), so one could (and some do) stream voice from there, however its one sided. Users have taken to using 3rd party applications such as Skype, Ventrillo, and TeamSpeak for voice chatting. There is also a user created program that uses LSLs ability to use email and XML-RPC and a downloadable program that creates a feeling of voice chat within SL. Linden Lab is said to be working on built in voice chat, but is waiting for voice masking technology (as well as other technologies such as speech to text and text to speech) to be not so crappy.

There:
There is currently the only virtual world out of the three that has voice chat built in. Voice chat can be used by pretty much anyone (who can get it to work properly) to talk to anyone around them with their microphone. Avatars mouths move and make hand and body gestures along with your voice. A graphlike bar is displayed over your avatars head when you talk showing the wavelengths and has redbars when your voice goes too loud or staticy. Some users have issues with getting There to work with their mic, as There's voice trainer software for the microphone can be a pain in the ass to get to work nicely with your mic (it seems to range from too sensitive to not sens sitive enough).

Active Worlds:
AW offers many different sorts of private world server packages. The packages range in size and how many maximum users can be in a world at a time. A user pays for an initial start-up fee and then for a anual fee to continue having the world in the world list, an additional fee is required if you wish for the world to be available to Tourists (trial users). These fees do not cover hosting, you must find a hoster for the server software, there are many user run hostings strictly for world hosting (I believe one or two free services). The fees go from 10$ to 1150$ (in USD) for worlds. Active Worlds also offers what they call "Solserver", "Galaxy", and "Universe". I won't cover these here, but more information can be found Here. Private worlds allow the user to set *many* options for the world, customization is almost unlimited, you can of course use custom models for objects, avatars, custom sounds, textures, etc. Private worlds also let you set stuff such as textures for land/sky/sun/water, gravity, fog, water height, build rights, and more.

Second Life:
SL has a few different options as well. While SL doesn't offer "worlds" it does offer private islands. These private islands are regions which are seperate from the main continent and allow the user to do almost whatever they like with the land (some restrictions do apply). Users can set a wide range of features from other user access options to sun position, land textures, and a few other settings. Private Islands however do not have as wide a range of options as AW's private worlds do. Private islands allow an almost unlimited creativity for the owner(s), they can be group owned for group projects as well. Another option is to buy an island/region/sim that is connected to the main continent through auctions held by Linden Lab on the SL website. These regions however do not allow you the same control as Private Islands do and more often than not some of the land is owned by Lindens, basicly its just like owning a very large ammount of land. The benefits of owning a sim connected to the continent depends on what your goal is for the sim, same applies to owning a Private Island.

There:
There does not offer private servers. However Forterra, creators of There, is supposedly working on a virtual world platform that would enable companies to create their own worlds based on the There platform. Forterra has already developed a world ("Earth") for the army (as seen on the Forterra website) which uses the platform, which is the same technology There uses of course. So at some point someone may be able to have a private world through Forterra, however the platform will most likely cost *alot*. Side Note: There is no longer owned by Forterra, a company made by former There Vice President Michael Wilson called Makena Technologies, Inc. purchased exclusive license of There, allowing them to further develop it.

Active Worlds:
Only offering one type of chat command, which is the "/me" command. This allows users to do custom "emotions". It shows up in the chat as "Name smacks you with a large trout" as opposed to "Name: smacks you with a large trout". Unfortantly this is the only chat command/gesture, other gestures/animations are available through button clicks on a toolbar.

Second Life:
A few different options are available. The IRC chat command "/me" is available in public chat (currently does not work in IM however), as well as custom gestures which can be setup via a Gesture window. The Gesture window allows you to assign any trigger words to trigger the gesture, such as "cow" or "/cow". Also one can talk on "channels" via chat commands, typicaly all avatar chat takes place on channel "0", but there are several million (negative and positive number) chat channels that are usualy used for scripting. Avatars can talk directly on these channels (mostly used for issueing script commands silently) by typing something like "/3 hello", which would cause "hello" to be said on channel 3, the user could then type "//hi" and "hi" will be said on channel 3 again (// causes the last used channel to be used again)(this also only works in public chat, not in IMs).

There:
Using a slightly different format of chat commands, There uses ' instead of / before commands/gestures/animations. These are built in by developers and use keywords. For example one could say "'lol" and their avatar will laugh. Some emotes can be emphasized by adding more 's, example being "''lol". These will show up in the chat bubbles, however you can hide the emote text by adding a ' after the text, i.e. "'lol'" will then appear as ['lol'] to you, but no one else will see it. There are many different words that can be used which will do different animations. There is no way to do "/me" commands.

Active Worlds:
AW allows you to use streaming video, also audio I'd guess. Special objects can be used along with the media command to stream video. In order to see video you must have Windows Media Player 9 installed. Users then activate the stream by interacting with the object in some way and it will play on the objects surface. More information about the media command can be seen Here.

Second Life:
SL uses the Quicktime library to enable streaming video, as well as a different setup for audio. Many formats that Quicktime supports can be streamed into SL, but not all, such as those that require interaction and some video formats, this may change letter. Audio alone streaming uses its own built in method and does not require quicktime. Whats the difference? Streaming Audio came first to SL and allows land owners to load a URL for their land to play the audio to anyone who happens to be over that land, supported formats are some sound formats and stream URLs. Streaming video was released in the newest version of SL, it too is based per land parcel. You can only have one audio or video streaming playing at a time, however you can have both an audio and video playing at the same time (you can't have two different video streams on the same land). Both have LSL functions which allow you to do tasks such as load URLs or start/stop/etc. video. Streaming Video works by replacing a certain texture which the land owner selects in the land preferences window, it will then replace *all* visible instances of that texture regardless if it is on the land parcel or not. Video is client side, so different people may see different parts of the movie based on how fast it streams or when they come in during the stream. There is much more information available on the forums.

There:
There lets you stream audio by placing out special objects and editing their preferences to set the streaming URLs. These objects can of course be bought in the store, usualy they are in the form of boomboxes or juke boxes. Only the owner can set the streaming URLs. Each object has a range and anyone within that range can hear the stream, users can turn off the stream by finding the object and looking at its Info window mute it. Multiple objects can be setup in an area, the closest one to the avatar will play. Objects can only be dropped in PAZ or houses of course and cannot be mobilized. Music can also be streamed over voice chat of course, allowing people to become walking DJs for anyone to hear. I am not sure what formats are supported, I suspect standard streaming links. I'm guessing There also uses some in-built system to play the sound. Currently There does not support streaming video.

Active Worlds:
Active Worlds is open to all ages. AW does this by giving every world a rating, using a system similar in style to Movie ratings, ranging from G to X (G, PG, R, X). G worlds are usualy heavily administered either by bots or volunteers to make sure nothing bad goes on. You can set via the client which rating you can go to and see (such as if you set R, you can see G, PG and R worlds, but not X), you can also password protect this to prevent a child from changing the setting.

Second Life:
Second Life is divided into two "grids" (or worlds), one grid is for children 13-17, the other is for adults 18 and over. Where you go depends on the born date you put in when signing up (sign up requires a Credit Card still). When a resident on the "Teen Grid" turns 18 they are automaticly sent to the "Adult/Main Grid", accounts still in tact (other than Calling Cards I think). No adults are allowed in the Teen Grid and no teens are allowed in the Adult Grid and no direct cross communication is allowed, if such things are found out to have happened the accounts involved will be permanently banned. The Teen Grid is rated PG completely, with no Mature or X rated areas, any Mature content found on the Teen Grid will be deleted immedietly and is against the Teen Grid TOS/CS (action will be taken). The Adult/Main Grid has two different ratings PG and Mature, some "Regions"/"sims"/"areas" are rated PG and some are rated Mature. In Mature areas one can express themselves freely with swear words and participate in sexual acts or have pornography but only in-doors, no porn or sex is allowed to be visible to "the outside world". In PG areas one can not use swear words, be naked, or have any pornographic material, if a resident is found to be doing any of these action will be taken.

There:
To enter into There you must be atleast 13 or over. I believe if you are under 18 you are required to get parental permission and to sign up you need a Credit Card anyway so it's a bit of a given. You may also be allowed to enter There if under 13 with parental supervision, however I am not sure about this and could be wrong. All of There is rated PG (there are some "skimpy" clothes, but nothing too risque), there are no "Mature" areas and avatars can never be naked (unless using a user-made third-party patch). Some users have taken to cleverly positioning couches and other furniture to create sexual poses, this typicaly does not happen anywhere in public however for fear of punishment.