For FPS, try America's Army.
Similar to CS, but with better graphics and more realistic gameplay (ie the sniper scopes and iron sights actually bob around to simulate breathing).
Free too (as in FREE BEER). Developed by the US Army to give civilians (an "inside look at the army" *cough* recruiting tool *cough**). Still a great FPS though. A lot of the members are actually members of the military as well.
Over 8,000,000 users at time of posting.
It's currently based on the Unreal Engine, but their currently developing version 3.0 (currently at 2.8) and it's gonna be completely redone.
http://americasarmy.com/
2.8 gig download though.