Midi Studio - Full-featured, modular MIDI control surface for iOS
Captain Chords - Write chord progressions in your favorite DAW
Navichord - Intuitive chord sequencer and MIDI controller for iOS
RapidComposer - MusicDevelopments RapidComposer: Music Prototyping and Automated Composition Software
IsoPad - IsoPad lets you use your Windows tablet to play electronic MIDI instruments.
Cthulhu - VST MIDI-FX plugin for harmonic inspiration