Tremulous

Tremulous is a free and open source team-based first-person shooter with real-time strategy elements. [1]

It was released on March 31, 2006. The game features two teams, humans and aliens, each having to build and protect a base. Victory is achieved by the destruction of the enemy base and by eliminating all members of the enemy team.[2] The game has been downloaded over 1,000,000 times[3] and was voted Player's Choice Standalone Game of the Year in Mod Database's Mod of the Year 2006 competition.[4]