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
Java decentralized cross-platform network multiplayer game engine
Cross platform 2D/3D game engine written in C++11 & OpenGL ES 2/3
A generic C++ MUD server featuring a strategy-style hexagon map