http://www.buygamecdkeys.com/

Found this site while looking for Torchlight II for cheap. Got a key from them and activated it on STEAM for USD$14 instead of USD$20. Checked it out before purchasing as well and found it's legit. Give it a shot for cheaper games. I will say that I see one or more other games that happen to be going for like half what it's being sold for everywhere else. I'll be using this site for a lot of my upcoming games in future.