MelodyMorph is an iPad app (unreleased) for making interactive musical instruments and compositions. Just drag notes onto the screen, with color for pitch and shape for timbre, then tap them to play.
You can tinker with melody and harmony, make a melody intro a ring or a tree, or build a customized piano. Create playable representations of existing songs, take a tune apart and put it back together a different way, and explore harmonic relationships. Compose your own musical landscapes, make an instrument to improvise on a particular song, even modify your instrument as you play. It's a new pathway into musical creation.