Braid
Of all the game genres to make a comeback this year, who would've picked the side-scrolling platformer? At first blush Braid looks like a standard old-school Super Mario Bros.-style game. But there's a twist: on top of standard game mechanics like running and jumping, Braid adds the ability to manipulate time: if you make a mistake you can wind back the clock and try again. It sounds simple, but as Portal proved last year, a simple mechanic in the hands of a brilliant developer can yield near-infinite entertainment. [src]
It's available on Windows, Mac, & x360 live arcade.