Slow on posting this, but forgot to mention two other notables I got off the Steam Holiday Sale:
A) Indigo Prophecy, some kind of horror adventure game that starts with a pretty cool premise ("I just murdered that dude! But why?") and by the end apparently degenerates into hobos vs. aztec gods and general trainwrecks. Haven't gotten too far myself, but it was only 3.50. (Now it's 10, which is still reasonable.)
and more pertinently...
B) Plants vs. Zombies, also for 10 (was 7.50). If you have any love for tower defense games, it's definitely recommended. Excellent sense of humor, and they diligently mix things up with tons of mini-games, puzzles, variant-modes, a special survival mode for after you've beaten the game, etc. Also: it has the CUTEST ZOMBIES EVER. I defy you to find any cuter zombies. The demo's free if you want to try it out (I'd played the demo in October, but only got the full game recently).