Solar2D Game Engine main repository (ex Corona SDK)
Powerful, mature open-source cross-platform game engine for Python
Open source clone of the Age of Empires II engine
RPG Maker 2000/2003 and EasyRPG games interpreter
Urho3D Game engine
2D game development library for Ruby and C++
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Free and Open Game Programming Language for Windows and GNU/Linux
Blueshift is a cross-platform 3D game engine implemented in C++
2D physics engine for games
A 3D, fully concurrent game engine
Cross-platform 2D gaming middleware