Red Eclipse
Play it. Nuff' said.

[SIZE=3]But seriously, it's a ton of fun. It's a fast paced first-person-shooter that has some resemblance to the Halo series, Quake series and Tribes: Ascend. It runs really well and has little to no lag issues, and is really friendly to low-end PCs. It's also on Linux and Mac. Best of all, it's FREE!
Surprised that it's not really well-known. It needs more love.
[/SIZE][SIZE=3]

[/SIZE]