Despite the epic Steam sale going on, I have yet to make any purchases.  At all. 
 
I am opening this up to the GB community, hoping for some recommendations to help me cut through the mass of games on sale. 
 
Guidelines/Info 

1.  I play on Mac, so it has be Mac supported. 
2.  Its a MacBookPro, 2.2Ghz, Intel Core 2 Duo type.  Has a GeForce 8600M GT.  I take pretty good care of it, but it isn't brand new.  Runs Half-Life 2 just fine, but struggles quite heavily with TF2...
3. Check my current library on GB's achievements page
4. I will also be posting both specific questions as well as accepting general recommendations.
5. Don't just link me to GB, Gamespot, MetaCritic, etc reviews, I can read those myself, thanks.
6. I don't want to just hear "This game roxxorz my soxxorz!"
7. If you have Steam, and are incredibly useful in your recommendations, I will gift you a game, limit of $15 USD.