Outkast - The Love Below
Track 17
Though I don't really see it as a hidden track, but the song is just an instrumental an is Untitled. I think the one who did the piano was Norah Jones cause the she was featured in the next song (Track 18 - Take Off Your Cool).
N.E.R.D. - Fly or Die
Track 07 - Wonderful Place
Track 08 - Drill Sergeant
Track 12 - Chariot of Fire
Each song has a song new after the first one but is only one track...