Antichamber is a game created by Alexander Bruce. As a first person puzzle game with a lot of white featureless walls, it bears a superficial resemblance to Portal, but differentiates itself pretty quickly. There is no GLaDOS, no artificial distinction between one puzzle and the next, and of course no portal gun.
What you get instead is infinitely more alien. Concepts such as euclidean space and object permanence break down as you progress through the game. In place of a passive-aggressive AI that wants to kill you you get doodles that when clicked provide one sentence life lessons that may or may not be related to the puzzle. As you pass through the game your expectations become more flexible. You no longer think that back tracking will work all of the time and you know that where you look can be as important as where you stand. The environment combines cubist style with Escher's sense of space. Combine this with eclectic sounds (insects chirping, wind blowing, rain falling) randomly interspersed in places leaving the player to wonder where they are as the dissonance between sight and sound scratches at the back of the player's mind.
The result is a zen acid trip that will keep you guessing. A koan become game.