OMG, The Stanley Parable. I haven't smiled, chuckled, or been surprised by a game this much since well...both Portal games. It's fantastic. Totally worth it. I wish I could say more, but it's something you need to experience yourself
I love how at first it seems to be only a smartly conceived, witty commentary on games, but as layers drop one after another it becomes apparent it's more of an observation on the nature of free will (or lack of it) and choice in a broader sense.