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.
The project 'MultiVerse' will be an open, decentralized, 3D virtual world where players and server owners can create or import content, build objects and define custom behaviour using scripts. Players can interact with objects in various ways.