I'm at least two years behind on this, I think, but Braid (braid-game.com) is a pretty amazing game. The entire game mechanic is based on going backwards and forwards in time, or slowing time down in order to solve puzzles. All of the puzzles are extremely clever, and some are damn near impossible.

Probably what struck me most was that it also has a compelling, albeit short story to it that plays extremely well into providing a narrative to what would otherwise be repeatedly making you feel not smart enough.

If you've wanted to check it out, please do. It's well worth the time and money.