I played the Portal closing-credits song, "Still Alive," for Kam the other day, as a sort of coda to our period of playing Portal. (Actually, I showed her the whole credits sequence of the game, 'cause seeing it adds to the experience of the song.) And of course the song stuck in my head again for hours afterward. I kept having to sing little bits of it to myself.
I'm especially fond of the line "Aperture Science: We do what we must because we can"--such a perfect corporate slogan, equal parts bland, ominous, and nonsensical.
I'm making a note here: Huge success. Jonathan Coulton is brilliant, and it really is the perfect ending to the game.
(Added later: I should have noted that the song probably isn't nearly as entertaining to people unfamiliar with the game, even if you know that it's from the point of view of a semi-deranged AI. And it arguably contains spoilers for the game, too.)