Warzone 2100 was one of the first 3D RTS games. It was released commercially by Pumpkin Studies in 1999, and released in 2004 under the GPL. The Warzone Project wishes to make Warzone 2100 one of top free software games available.
In Warzone 2100, you command the forces of "The Project" in a battle to rebuild the world after mankind has almost been destroyed by nuclear missiles. The game offers campaign, tutorial, multi-player, and single-player skirmish modes. An extensive tech tree with over 400 different technologies, combined with the unit design system, allows for a wide variety of possible units and tactics.
Can be downloaded from: http://wz2100.net/download