One framework for creating powerful cross-platform games
Open Source MMORPG Framework
CTFs as you need them
You can make it - Open Source MMORPG Platform
Arianne is an engine to develop multiplayer online games like Stendhal
JavaScript framework to build game engines, games, emulators and apps
Java Settlers
Cross platform 2D/3D game engine written in C++11 & OpenGL ES 2/3
C++ game libraries plus server.
Cross-platform reimplementation of AMOS BASIC (runtime)
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