Glest: a free 3D real time strategy game available for several operating systems!
Superseded by Mega GlestGlest is a free 3D real time strategy game, available for several operating systems and that can be modified using XML and a set of tools
The core game allows the player to take control of two completely different factions: Tech, which is mainly composed of warriors and mechanical devices, and Magic, that prefers mages and summoned creatures in the battlefield.
http://www.glest.org/en/