Two words:
The Polynomial. You fly around a 'space' environment created by fractals that pulse to whatever music you put in (or a rad accompanying soundtrack), shoot at blacklight-and-neon pacmen, and save jellyfish. If you die, you respawn instantly. It's a beautiful experience, not too complex, and it zens you into some satisfying-but-not-too-consequential action. It helped me relax at the end of the day during some very, very rough times last year.
There's a highscore that tracks your last 1, 5, 20, and 60 minutes of playtime, but I prefer to simply enjoy my pacmen-shooting and intergalactic jellyfish-saving. There's also a free-fly mode wherein you can remove the enemies, and in whatever mode, if you ever have enough of one particular environment, you can just wormhole over to the next. Watch the trailer and you'll see what I mean. Ooh, and it's also $3.40 during the sale right now. Highly, highly, highly recommended.
http://www.youtube.com/watch?v=BQfYc9UgOJo