And I don't even own a 360!
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It's the end credits song for Portal, the newest superhit sci-fi game in which, if I understand correctly, you are a human lab rat testing a new technology for an insane AI who promises you cake if you get through the whole thing without dying.
The song, of course, is sung by the insane AI. It's written by the brilliant Jonathon Coulton, and if you know his work you shouldn't hesitate to watch.
Between this and Beautiful Katamari I officially want a 360.
Someday.