Going through a list of my most played games, and excluding those that are particularly well known (Like Borderlands series, TES, Saints Row, etc.), I'll recommend:
Beat Hazard - Nice twin stick shooter that uses your music to generate on-screen objects.
http://store.steampowered.com/app/49600
Dragon's Dogma: Dark Arisen - Though it only has 33% off it's a great RPG. Nearest similar sort of game I can think of is Monster Hunter, though I haven't played much of that series.
Prototype Franchise - Nice bloody action games.
Unreal Tournament Series - Specifically, I rather like the original. The later games have their fans, and UT3, while about a decade old, is still alright as I recall.
Disgaea PC - Yeah, it hasn't been out on PC long, so the discount isn't much (25%), but it's one of the best Tactical Strategy RPGs out there. Well, it's pretty deep, though nowhere near as much as the later games in the series. The more that buy this, the more likely the later titles will make it.
Magicmaker - A fun little game where you mix and match magic 'ingredient's to create interesting, and sometimes powerful effects.
Recettear - I can't recommend this game enough. One part shop management, one part dungeon crawler, and all sorts of charming and amusing. It never got a sequel (spiritual or direct), which is a shame. I would pay all the money for one.
Risk of Rain - An platformer with some action mixed in. A half dozen or so unlockable classes. It's not easy, but it's pretty good.
Rogue Legacy - A 'rogue-lite' action-platformer game where you suffer permadeath with a twist...Your descendant takes over. This is one of like, two Steam games I ever got all the achievements for, so I definitely like it. SEQUEL PLZ!
Kingdoms of Amalur: Reckoning - More famous for the horrible mis-management of the studio which lead to bankruptcy. This is still a pretty great game, though it doesn't seem likely we'll get a sequel. It plays a bit like a MMO, with several classes to choose from.
Well, that should tide you over, and maybe rob some of you of your cash, muahahahaha.
I'm totally not an agent of Gaben, honest!