Tremulous is a free, open-source RTS/FPS hybrid that once again pits teams of humans versus teams of aliens in mortal combat over the last piece of apple pie. The RTS aspect of the game comes from players being able to build helpful structures during the game (like respawn points). It looks pretty cheap, but could be fun for fans of this sort of thing.
Also, the creators are quick to point out that they were working on their game long before Natural Selection, so there!
(Source: Kotaku)