Ok, some thoughts on things I already own and recommend:
First, Defense Grid is crazy fun if you like tower defense type games. Between playing the actual games, and the medal achievements, I put a fair number of hours into it.
Batman Arkarm Asylum is probably the best comic book superhero game I’ve ever played. It’s a 3rd person fighter in an open world, and it’s really addictive. I don’t normally go in for the spiderman/superman type games, but this is much better than any of those games ever were.
Braid was fun for a hour or two, but I got bored. it was really pretty an innovative, but not my style. Again, for the price… check it out.
Stalker was a moderate amount of fun. For $2… why not.
The original Deus Ex was probably the best game out when it was released. if you’re into classic fps games, it’s worth a look.
I loved the XCOM games when they came out. Since my copy doesn’t work anymore, I picked these up for nostalgia in case I ever want to. And if you enjoy them, the Civ III and Civ IV packs are pretty cheap. I picked up the Civ IV pack a little while ago just to avoid hunting down all the CDs when I wanted to re-install it.
Rome: Total War is totally worth $2.50.
Torchlight is an amazing indie game. It plays like Diablo, and it’s well worth $10.
Ghostbusters should have been good. But it’s not. I’d avoid it.
For newer games, Borderlands was a lot of fun as a shooter. The DLC for it was OK. Not great, but not bad. Obviously the Left 4 Dead games have a big Steam following around here and I enjoyed them quite a bit. BioShock was ok. Not my thing, but I have a lot of friends who swear that it’s the best game of the decade and it’s fairly cheap. GTA IV for $7.50 is worth it assuming the bugs are out of the PC version. I loved it on the 360. Not as good as III, but still decent enough.
If there are any other games you’re on the fence about, ask. I’ll give my opinion if I’ve played it.