Flowkey - The easiest way to learn piano with your iPhone or iPad
Chordsheet Maker - Create chord sheets or lead sheets quick and easily from your iPad. Add rhythms, endings, segno, codas and all what's necessary for your musicians to know what's your music about. Transcription made easy!
alphaTab - alphaTab is a cross platform music notation and guitar tablature rendering library.
Tonelib JAM - Create, edit, print and listen to tablature and guitar sheet music
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
Aria Maestosa - Aria Maestosa is an open-source (GPL) midi tracker/editor.