Desktop/Android/HTML5/iOS Java game development framework
You can make it - Open Source MMORPG Platform
Open source library for creative expression on the web, desktop, etc.
Gideros Release version
Arianne is an engine to develop multiplayer online games like Stendhal
PyCAPGE - Python Classic Adventure Point and Click Game Engine
An API for interacting with Starcraft: Brood War (1.16.1)
Open-source, cross-platform, C++ game engine for creating 2D/3D games
An extensible development framework for roleplay games.