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.
Over is a space-based comic-like RTS game with some MMO features (persistency, commander profile, etc...) It also features customizable fleets and wings. Over is build on a completely custom engine which currently runs on linux and windows.