The main complaint that usually arises about open source games, and specially FPS, is that the graphics aren't good enough and that they clearly are amateur projects. But now and again a project like Tremulous is launched surprising everybody.
Choose your side: Alien or Human
Tremulous is an online multiplayer FPS game which evolves around the confrontation between aliens and humans in their fight for survival on board a spaceship. The objective of this game is none other than to eliminate all the enemies.
Make sure your enemies can't respawn.
Unlike other FPS, in Tremulous the players will have the possibility to increase their characters' skills and buy new equipment, something that in the long run will help them destroy their enemies' structures, some of which are in charge of allowing the characters to respawn.
One of the most important elements of Tremulous is the graphic quality they have, because to develop it, the team has used the id Tech 3 graphic engine, which was originally developed for Quake III Arena.
Therefore, if you're looking for a game with which you can have a good time annihilating the enemy, you only have to download Tremulous for free.