I haven't been much of a gamer, but I recently purchased Allied Assault and have become quite addicted to it. I was amazed to find that there are completely open source FPS out there, though from what I understand, the quality isn't up to current proprietary standards.
From what I can see of the movies, though, the gameplay isn't too bad.
Here are some links to check out if you are interested:
http://www.nexuiz.com/ - a FPS http://www.planeshift.it/ - More of a RPG than a FPS, but looks neat.
Enjoy! Josh