Captain Chords - Write chord progressions in your favorite DAW
TouchDAW - Full-featured DAW control and general purpose MIDI control app.
RapidComposer - MusicDevelopments RapidComposer: Music Prototyping and Automated Composition Software
XotoPad - MIDI instrument controller with chords, scales, faders, and more for multi-touch Windows devices.
Cthulhu - VST MIDI-FX plugin for harmonic inspiration
Midi Studio - Full-featured, modular MIDI control surface for iOS