The RML project is working on a way of designing rulesets for games, virtual worlds and other simulations. It aims to enable design at a much higher level than is currently available and make it accessible to the more artistically inclined designer.
A codebase for the construction of 3d online virtual worlds. Includes everything you need to make your own multiplayer online roleplaying game. All game rules are contained in plugins which can be written in easy scripting languages such as Python.
VTSystem is a 3D (Action) Adventure engine where each character can go about his(her) own affairs independent of your actions, where you can experience 'real world' environments! Uses the Nebula Device. For more information, take a look at the homepage
A graphical isometric multiplayer virtual reality server in MUD-style. Rooms and server code can be changed online. At the moment, there is also an IRC link. Clients are available in Java and Python/C.