I am addicted to Steam sales.

I check twice a week for the mid-week and weekend deals even though I don't have time to play any games at the moment until I've finished renovating my house

So what was the best game-playing-value-per-dollar deal you got on Steam? One game (or game pack) per answer please (don't forget the price).

I'll add my own to get it started...

Update: Now with added community wiki goodness!