Open source clone of the Age of Empires II engine
Powerful, mature open-source cross-platform game engine for Python
3D game engine/framework in C, with Luajit and Python bindings now
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
GemRB is a portable open-source implementation of the Infinity Engine
DOOM / Heretic / Hexen source port with enhanced graphics and UI
KlayGE is a cross-platform open source game engine
The second demo of my game engine.
Tactical, turn-based game engine, that uses OpenGL and WinAPI
Open source, moddable game engine for the "Polygon Love 2" game
The Lambda-Engine is a cross platform game engine.