The two well know open source multiuser VRML systems are VNet and DeepMatrix. VNet created by Stephen White and Jeff Sonstein. DeepMatrix is being developed by the folks at Geometrek. Both systems are being actively maintained and enhanced.
VNet has been around since March 1998, and matrix the predecessor of DeepMatrix was released a short time after also. The big deal of both these systems is that they both use regular, good old fashioned VRML browsers. Just fire up CosmoPlayer or WorldView and head off to some sample worlds.
Speaking of sample worlds make sure to go to Sonstein's VNet home page which contains links to all the "big" VNet sites. In addition it's worth noting that Jeff's vrmLab Town Square has according to him been running since VRML began. Well I don't know exactly about that but I do know I've been visiting there off an on for a LONG time.
VNet supports both avatars, those funny little characters used to represent yourself in a shared virtual environment and shared behaviors. That way if you are waving your hands other people in the environment can see that you are waving your hands.
Some of the environments are quite complex an show off excellent VRML world building. In particular check out Forest World by Niclas Oloffson (back up shortly) and Electra City by Miriam English. On an amusing side note Forest World was invaded by virtual squatters, created by Cati Laporte, complete with grimy surroundings and rats.
DeepMatrix is an attempt by Geometrek to create a open source multiuser system, and actually sell services for using it. Geometrek will be happy to customize avatars and worlds for your DeepMatrix site. They do provide a few "off-the-shelf" worlds and avatars.
Geometrek is really a virtual organization and many of the people associated, live far from each other and support Geometrek remotely, however I can't emphasize enough that some of the best VRML people out there are associated with Geometrek.
Some of the multiuser worlds really demonstrate the potential for shared multiuser environments. You can place a game of chess, work collaboratively to solve Rubik's cube, or just hang out on the beach (I wish!). There's a fairly extensive collection of avatars, objects and landscape to help you roll your own.
Happy trails, and see you in cyberspace!
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