SL, ActiveWorlds? Old School ... New Age is Project Wonderland!

Project Wonderland: Toolkit for Building 3D Virtual Worlds
Experimental Technology sponsored by this community and Sun Microsystems Laboratories
https://lg3d-wonderland.dev.java.net/index.html

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Project Wonderland is a 100% Java and open source toolkit for creating collaborative 3D virtual worlds. Within those worlds, users can communicate with high-fidelity, immersive audio, share live desktop applications and documents and conduct real business. Wonderland is completely extensible; developers and graphic artists can extend its functionality to create entire new worlds and new features in existing worlds.