Alien Arena is a FPS that has been free ever since the humble beginnings in 2004, and it looks like it will continue to stay free. You can play against other people all over the world, or in offline mode against computer controlled Bots. There are 46 levels to rage war on, and you have 5 characters and 9 weapons to choose from. You can play the regular Deathmatch, Capture the flag, Deathball, Team core assault, Team Deathmatch, All out Assault and Cattle prod. In the last one, you are supposed to herd a group of robotic cattle from A to B.
As all FPS game, the game is quick and actionfilled and can sometimes be intimmidating to beginners. But stay with it, and you'll soon get the hang of it.
Alien Arena can be downloaded for both Windows and Linux, and it is said that it is possible to play it on a Mac by donwloading the linux version and then applying this patch to it.