Arianne is an engine to develop multiplayer online games like Stendhal
JavaScript framework to build game engines, games, emulators and apps
Game engine supporting many 3D/2D formats and graphic effects
Java Settlers
3D .NET engine for Windows, Linux, Mac
Free strategy browsergame engine.
AutoTeach makes kids study and learn in exchange for internet access.
C++ game libraries plus server.
Modular Gaming Open Source Persistent Browser Based Game Framework
Open source network game service
Cross-platform reimplementation of AMOS BASIC (runtime)
Game framework on top of Python, Panda3D and Twisted
Atlarius is no longer under development.