Thomas was Alone. It only takes a few hours to complete, but damn is it a good few hours. I have never been so attached to a bunch of rectangles before. If you're a fan of platformers, and love an excellent, simple story, then get this game.
Little Inferno. Another game that's only a few hours long. Basically, you burn stuff, collect the coins that pop out, and buy more stuff to burn. It has a nice story attached to it, and is actually quite a good one at that.
Cthulhu Saves the World and Breath of Death VII. Plenty of good humor, solid mechanics, and at the low, low price of $2.99 for the combo pack(on sale at the moment for $1.49), they're a steal. At the same time, I'll recommend Penny Arcade's On the Rain-Slick Precipice of Darkness 3&4. They're both made by the same guys who made CSTW and BoD. I haven't played Precipice 4 yet, but from what I've seen, it's as solid a game as 3 was.
FTL: Faster Than Light. A sorta rogue-like in space. Each run can be anywhere from a few seconds to about 20 minutes. It's pretty fun, but can be rather hard at times, especially on Normal(there is no hard mode because that would be sadistic). It can sometimes be a bit too random, especially when trying to unlock the Crystal Cruiser. Overall it's a great game, if a little frustrating at times.
Super Meat Boy. Do you like hard as balls platformers? Does the idea of a little meat man being chewed to bits by a rotary blade pique your interests? Does the sight of blood make you all warm and fuzzy inside? Then get Super Meat Boy, and discover the wonders of high blood pressure!

Joking aside, SMB isn't actually as ball-bustingly hard as people make it out to be. It is hard, but once you get used to the controls, and become a wall-jumping god, beating each level just takes patience. Oh, and it's recommended to play with a controller. I play with keyboard because fuck the police, but it's much, much easier to play with a decent controller(my controller is a piece of shit >.>)
I'll second both Portal games. They're both quite fun and only take a few hours to complete.