If you have an XBox 360 and $15, you should download and play Braid. If you don't have $15, you should download the demo. If you don't have an XBox 360, you can wait for it to come to PC later on.
Or, you can make friends with an XBox 360 and $15.
I first saw a version of Braid at the Game Developers Conference several years ago - it's been in development for four years, at least. Unfortunately, one of the first things I saw was the last level, so the "punchline" has been spoiled for me, but I'm still really pleased with the journey to get there.
It's beautiful, which is something, but the reason to play it is that the gameplay is unlike other games. Like many platform games, gameplay is broken into a number of worlds, and within a world, multiple levels. Within a world - and this is the interesting bit - you can manipulate time in some way. It's different from world to world, and I won't spoil much, but in the first world you explore, by pressing the X button, you can rewind the clock, which is introduced as the mechanism for working with player death. Oh, I died, I'll just rewind to a place where I'm not dead, and then play on. But then it gets to be an integral part of gameplay - some pieces of the world don't rewind when you press X, and managing the progress of time along two (or more?) timelines is a rich domain for gameplay.
I have a friend who has already finished the game. And I've read online that it's not as long as people would like, but that's what they tell you in stand up comedy school - leave them wanting more.