Sampulator - Make (and record) beats on your keyboard
ChordU - Extracts chords from any song, integrated YouTube.
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
Guitaa.com - Turn ANY song into chords, play along with interactive chords and diagram, transpose, loop, tempo control.
Ramsophone - A generative art/music machine. (Be sure to refresh!)
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.