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
Java decentralized cross-platform network multiplayer game engine
C++ game libraries plus server.
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
The next generation mud is an open-source extensible, programmable mud