Metal Factions is a FREE 3D real time strategy game that runs on the Spring RTS engine.
It's an evolving game that currently features four factions: AVEN, GEAR, CLAW and SPHERE.
You can find more information and how to download and play on "http://www.metalfactions.pt".
You're not allowed to redistribute modified versions of the game itself (CC-BY-ND), but can use many of the modules for your own purposes (namely the ones on Luarules and Luaui which have GPL or PD licenses). Check the LICENSE.txt file for details.
To learn more about the structure of the game package, consult the engine wiki: https://springrts.com/wiki/Gamedev:Structure.