Counter-Strike: Condition Zero, the expansion to the original Counter-Strike, was recently made available for Linux on Steam:
Counter-Strike: Condition Zero (CS:CZ) is now available for Linux players via Steam. CS:CZ is the fifth Valve title to be released for Linux and, with its release, pushes the total number of Linux games available on Steam to 80.
Launched just three weeks ago, the Steam for Linux client is available for free from the Ubuntu Software Center. More Valve titles are heading to Linux in the coming weeks and months.
Condition Zero is significant due it being the first time you were able to play Counter-Strike in Single Player versus AI competition. I enjoy it because the AI is a little bit easier to kill than the teenagers who can kick my ass and then tea-bag my corpse.