Green Man Gaming is selling Metro 2033 for $3.74 for 24 hours leading up to the release of Metro Last Light tomorrow. This is a series I have heard a lot about and it sounds very interesting to me. I had it in my gamefly queue because I wanted to play it before I play Last Light. At less than $4 I'll just buy it though. When you buy it from Green Man Gaming they just give you a steam activation code for the game.
http://www.greenmangaming.com/s/us/en/pc/games/action/metro-2033/
http://www.greenmangaming.com/s/us/en/pc/games/action/metro-2033/