Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Spring is a versatile 3D RTS game engine. Using extensively Lua for scripting game-specific code to make nearly every aspect of the engine customizable, from GUI, to unit AI, to pathfinding.
CorEngine is a work in progress, OpenGL graphics powered 3D game engine designed to help independent game developers with quick prototyping and game/virtual environment creation.
The engine supports a standard set of features, like skeletal animation, post processing, Lua/C programming, physics powered by Bullet Physics, GUI and 2D/3D Audio.