Open Source MMORPG Framework
You can make it - Open Source MMORPG Platform
Arianne is an engine to develop multiplayer online games like Stendhal
Java Settlers
Open Source Virtual World and MMOG Development Platform
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
A generic C++ MUD server featuring a strategy-style hexagon map