TF2 is free-to-play now, but you should totally get that. Damn, just get the entire Orange Box, you'll get your money's worth.
How about telling us what games you already have and like so that we can recommend games that you would more likely to enjoy?
The Left4Dead's are really enjoyable (I like number one best)
Chime (cute little game, I've had a friend say it's Tetris for people with OCD ^_^ )
Plants vs Zombies (a smash hit, not undeservedly)
Borderlands (loads of fun, whether in single player or with friends)
Machinarium (a true gem, a complete masterpiece, but don't get it if you don't enjoy puzzles)
VVVVVV (if you like retro stuff; I do, I'm a bit of a nostalgic)
Braid (innovative and fun, challenging)
There are too many to list, but that's a start.
What you should really do, though, is keep an eye out for Steam sales. My motto is "never pay full price". ^_^ Happy gaming!