Alien Arena is a free first person shooter. It has a 1950's science fiction atmosphere. You can play online deathmatch or play single player mode against bots. The game engine is open source but other content is proprietary. There are over 50 maps that you can choose from and 9 different weapons. If you are playing online you can choose from deathmatch, team deathmatch, capture the flag, deathball, team core assualt and cattle prod. There are also powerups to collect. The game features nice graphics and fast gameplay. Check out a video of the game here. To download the game go to their website.
Alien Arena site.