Arianne is an engine to develop multiplayer online games like Stendhal
JavaScript framework to build game engines, games, emulators and apps
Java Settlers
Single-/multi-player card game Mau Mau (similar to UNO®)
Java decentralized cross-platform network multiplayer game engine
C++ game libraries plus server.
Cross-platform reimplementation of AMOS BASIC (runtime)
Game framework on top of Python, Panda3D and Twisted
The next generation mud is an open-source extensible, programmable mud
A generic C++ MUD server featuring a strategy-style hexagon map