This one is really cool. Backtracking part of the Stairway to Heaven song and you get satanic lyrics:
http://www.youtube.com/watch?v=BwHg5sCvs0M
Funny thing is though, I think it's an auditory illusion (much like an optical illusion).. like when you see faces/shapes in clouds, here your brain tries to make sense of the noise and you only hear the lyrics when you see them displayed in front of you. (if I close my eyes it sounds like random noise)
