http://www.warsow.net/
"Set in a futuristic cartoon-like world where rocketlauncher-wielding pigs and lasergun-carrying cyberpunks roam the streets, Warsow is a completely free fast-paced first-person shooter (FPS) for Windows, Linux and Mac OS X."
Warsow is one of the best open-source games I have ever played. It is a very fast FPS, with emphasis on the "art of movement." With the release of v0.6, the game got a face lift; new graphics, models and engine optimizations make the game even better. Some weapons balances have been applied to make the game more noobie friendly, as it has historically had a steep learning curve.
My advice: give it a try, and don't get frustrated too quickly. If you play enough, you'll be bouncing off the walls and flying through the halls at a million kilometers per hour, too.
If you're good with code, join the dev team. I personally don't have the coding skill for that, but I've contributed on the Wiki quite a bit. It's neat to know that you can help make the games you play.