Battle for Wesnoth is one of the very few Open Source games that doesn't look and play like a child made them in an afternoon.
It's a turn-based tactical wargame in the style of Heroes of Might and Magic and Age of Wonders but on a smaller scale. Good gameplay, decent graphics and *gasp* not all that shitty AI.
Armagetron is a loads of fun too.
NWN has a linux client. Since there are over 9000 modules out there for it, it'll be hours and hours of gameplay. Warning: main campaign sucks. Expansion campaigns are reasonable. Both are outdone by the modding community.