Oneiro - A text-centric, adventure game engine written in Java. [Open Source, GPL] - http://sourceforge.net/projects/oneiro/
Virtual Universe - More than only a simple 3d-chat: here a complete virtual reality is simulated. - http://www.3dchat.org/
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/
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/
The Millport project - Development site for a massive multiplayer game with a graphical interface, written in Java. [Open Source, GPL] - http://millport.sourceforge.net/
Worlds.com - Chat in a virtual reality 3D world. - http://www.worlds.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/