Bamba mentioned this in Bits and Bobs, but it deserves its own thread. I don't want to give too much away, as it spoils the game, but you should immediately play the demo at your earliest convenience. It doesn't have any content from the main game, so there are no spoilers there either. Then buy the game.
It fuses elements from Dear Esther, Go Home, Antichamber and Portal, but it's better than all of those except Portal. It's sort of the gaming equivalent of Primer or Inception.
It's brilliant and only £7.99 at the moment. Buy it now.http://store.steampowered.com/app/221910/