OpenArena, Warzone 2100, Urquan Masters, and Frozen Bubble should top your list of Linux games. Also check out Urban Terror, it is a close analog of CounterStrike.
The Wesnoth game I have never played, but am going to check it out right now
Urban terror - YES YES yes YES YES YES
Ive been playing this game since pre-beta on the mac , and its awesome , its a ET powered Counterstrike style game , But faster paced, plays well on hardware back to g4s and on intel machine with integrated graphics cards fairly well.
And it isnt your typical graphics game