Ad Infinitum
   Massively multiplayer game. [Open Source]
   http://sourceforge.net/projects/adi/

Building Virtual Worlds To Teach Archaeology:
   Discusses the creation of a virtual world to teach archaeology and the life of the ancient Maya to a target audience of middle school students.
   http://www.cs.ucf.edu/~moshell/CREAT/Caracol/TimeTravelRep.html

Fate
   Massively multiplayer space trading and combat game. [Open Source]
   http://fate.sourceforge.net/

Mars Simulation Project
   Open source Java project to create a simulation of future human settlement on the planet Mars. [Open Source, GPL]
   http://mars-sim.sourceforge.net/

Oneiro
   A text-centric, adventure game engine written in Java. [Open Source, GPL]
   http://sourceforge.net/projects/oneiro/

Tag3D
   Interactive Virtual World system using Java3D and RMI. [Open Source]
   http://www.croftsoft.com/portfolio/tag3d/

The MUGU Project
   Massively-multiplayer roleplaying game system, emphasising graphics over text. [Open source, GPL]
   http://mugu.sourceforge.net/

The Millport project
   Development site for a massive multiplayer game with a graphical interface, written in Java. [Open Source, GPL]
   http://millport.sourceforge.net/

Virtual Universe
   More than only a simple 3d-chat: here a complete virtual reality is simulated.
   http://www.3dchat.org/

Worlds.com
   Chat in a virtual reality 3D world.
   http://www.worlds.net/