The Binding of Isaac
Somehow I've managed to log 80 hours of game play into this game. This rogue-like dungeon crawler gives you control of Isaac and company as they make their way through the basement to fight Mom. Each completed run takes about an hour, faster if you're skilled and lucky, which isn't a terrible time sink.
Graphically this game runs off flash and the style is unique bordering on grotesque as there is a lot of blood, urine, fecal, and other similarly horrifying images. Despite the gore, there is a lighthearted and innocent quality to it all that keeps bringing me back.
Mechanically, the game is simple enough and straight forward. Although there is no gamepad support you can get around that with third-party software.
I look forward the the revamped version that's coming out in a couple weeks.