Open source clone of the Age of Empires II engine
Solar2D Game Engine main repository (ex Corona SDK)
Powerful, mature open-source cross-platform game engine for Python
Urho3D Game engine
2D game development library for Ruby and C++
RPG Maker 2000/2003 and EasyRPG games interpreter
A free 3D game engine to create games and realtimes scenes for Windows
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
RTSish Game Engine
Opensource Game Studio
Free and Open Game Programming Language for Windows and GNU/Linux
Blueshift is a cross-platform 3D game engine implemented in C++
Real-time RPG for PC/tablet/phone; multiple quests and random dungeons
2D game engine for traditional platform games
2D physics engine for games
3D Game engine
A 3D, fully concurrent game engine
Tactical, turn-based game engine, that uses OpenGL and WinAPI
A DirectX based graphics/physics engine
Cross-platform 2D gaming middleware