CS/S/CZ, TF 2, MW2 (never went past the second level of the campaign), DoD/S, WoW (Though you aren't counting it), BF 1942 + expansion, BF:V, BF 2, BF 2142, Fifa 09/10 (only play online or offline multiplayer), Killing Floor..I'm sure if I had a look through my games I could come up with some more.
See I think this is a silly question with all the TF2 people round here but eh what the hell right?
EDIT: Looking through at some responses, I must say it's quite silly to say it's not worth the money because it's essentially a long rental, I am 100% positive that I have put a lot more time (or essentially played a lot more) into each of the games I have listed (even the normal and source versions) than in any single player game (unless you're silly and count an mmorpg as single player).
I think multiplayer is what really makes a game worth its money unless it's something like LOZ, B&K, SSSV, HL/2, Viewtiful Joe, TES, all GTAs, any Mario game really, but while those have been more engaging, they have lasted a lot less time wise, and are thus arguably not worth the money by comparison.