I know it's been mentioned before, but I cannot stress enough how vital it is you wait until the Steam Holiday sale. There will be no point in buying games from them until that occurs.
Now, that being said, I'll lead things off by mentioning TF2 (again, mentioned earlier). It's the game that I have more hours invested in than any other game on my PC. You really have nothing to lose with a free game other than time, so absolutely give that a shot.
As for BF3: if you get it, you should join the Escapist group for that stuff; they're pretty cool folks. They just recently switched their voice stuff over to Mumble, so you might want to pick that up, again, if you are to get the game (which I'd highly suggest; it's so damn beautiful at higher settings).
Now, just a few random suggestions that you may or may not be interested in:
Atom Zombie Smasher
Bastion (please, oh please, buy this game)
Batman: Arkham City (the PhysX bits they added for nVidia cards is pretty nice looking)
Deus Ex: Human Revolution (and the original, while you're at it)
The Fallouts (1 and 2 if you're more of an old fashioned RPGer, 3 and New Vegas if you're more of a Skyrim guy)
The Elder Scrolls III, IV, and V
The Orange Box (because the Half-life games are fun, Portal's great, and you get the nice version of TF2 in all one convenient pack)
Just Cause 2
Mass Effect 1,2
Portal 2
Civilization IV, V (currently addicted)