Portal

[Piaresquared]

A truly amazing game. Right now on Steam you can play it for free until May 24th. I highly recommend it. It's a very short game, and probably won't take you more than 8 hours total. It took me 5½ my first time, and less than 3 my second time, but it makes you think in a whole new way, and the voice-acting and conception of the game is awesome.