The popular 3rd person shooter game Quake Live has come to Linux. It can be played for free by browsing to http://quakelive.com/ using Firefox. I played it last night and had a blast.
I really like the idea of the game going through the browser. I hope that HTML5 and on making building browser-based games without plugins a reality, but until then this is a great solution.
I have tried it on my Ubuntu 9.04 Jaunty 64-bit machine, and aside from some very small sound glitches, the game plays great. Tonight I'm going to give it a go on my laptop.