I am an active user of Steam, and recently, they put Red Orchestra on sale.
RO is a WWII FPS that is like none other. In my opinion, it is a perfect blend of realism and fun. Think COD2 on giant maps, vehicles, and a healthy splash of realism.
I love COD2 and COD4 to death, but in those games, you are basically a floating gun. RO definately pulls me in quite a bit more. The actions are very thought out, and the reactions to everything are just as good. You can be holding on for dear life during an artillery barrage, pop up to shoot, only to have your own weapon blown out of your hands, the hitboxes are very well put together.
The best part is that it is $4.99, and you get RO, which covers WWII on the Russian front, as well as access to the Darkest Hour mod, which covers the more traditional American side of the war. $2.50 for each game basically...
If you are interested, but you can't be separated from your precious cash, I have 2 guest passes which give you unlimited access to the game for a period of time.
Check this game out, you might be pleasntly surprised at what you get.