Bjork’s Biophilia iPad app and its first in-app release, Crystalline, are STUNNING.  Seriously, words cannot really do it justice.  You play a game where you collect crystals in certain sequences to unlock different arrangements and elements of the song, which you can then arrange in real-time while playing the game by choosing different tunnel.  I’m addicted, and Bjork is giving me something I’m willing to spend money on.