Unity3D Client And C# Server Framework
Arianne is an engine to develop multiplayer online games like Stendhal
JavaScript framework to build game engines, games, emulators and apps
Java Settlers
2D game engine for traditional platform games
Single-/multi-player card game Mau Mau (similar to UNO®)
Java decentralized cross-platform network multiplayer game engine
C++ game libraries plus server.
Open Source Virtual World and MMOG Development Platform
Create your own online real-time browser-based MMO in your website
Cross-platform 2D game engine
Game framework on top of Python, Panda3D and Twisted