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.
Open Source Virtual World and MMOG Development Platform
3D Client (Windows), Server, and Tools to easily create massively multiplayer games and virtual worlds.
If you want to contribute to the source code please go to https://github.com/multiversemmo/MultiversePlatform
GMUVE (Graphical Multiuser Virtual Environment) is an engine/framework that lets large numbers of users interact in a 3D world. GMUVE is a peer to peer networked OpenGL application that can be used for MMPOG or 3D chat. It is available in Python and JAVA