Team Fortress 2! It really is a great game, if you are looking for a very teamwork-based game, this is the one. The cartoonish graphics are fun and new.
Also, BF: Heroes might have been released (don't take my word for it, I haven't been paying much attention to it...) and FROM WHAT I SAW, it looked somewhat similar to TF2 (graphics, animations...) You can ride on the wings of airplanes!
Good luck
EDIT - I also recommend Fallout 3, I haven't played it on pc, but there is so much that you can do. Being the tons of items you can collect, bobbleheads, 'rare' weapons (or whatever you would call them).
The expansions are also great fun too, the only one I haven't played is Point Lookout, which is, I believe, about some infested zombie-like swamp thing?
That's my two cents, GET THOSE TWO GAMES!