Tonelib JAM - Create, edit, print and listen to tablature and guitar sheet music
Real Piano Teacher - Real Piano Teacher is a music learning application that lets you learn how to play the piano, organ, flute, Guitar, and many other instruments just in seven days.
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!
Flowkey - The easiest way to learn piano with your iPhone or iPad
Aria Maestosa - Aria Maestosa is an open-source (GPL) midi tracker/editor.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.