Tremulous is a free and open source team-based first-person shooter with real-time strategy elements. 
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. The game has been downloaded over 1,000,000 times and was voted Player's Choice Standalone Game of the Year in Mod Database's Mod of the Year 2006 competition.