The Antichamber logo

Antichamber is a non-Euclidian puzzle game, which basically means that it's good at messing with your mind.  In many ways, the game is like an indie Portal.  Both are very fun puzzle games that challenge the way you think about problems.

The basic mechanic is that the way you look at things affects things.  For instance, you might walk down a corridor, turn around, and notice that there is now a wall behind you so that you can't go back, but if you walked backwards into the same room, the wall wouldn't appear.  Similarly, some things disappear or change when you look at them.  There are also a few matter guns that you can get to create, remove, and move blocks.

Antichamber has a minimalist aesthetic, which it pulls off well, much like Limbo.  It is challenging, but it's fairly quick as well, and you usually feel like you're making progress.