La Touche Musicale allows you to learn the songs you love on piano. Connect your MIDI keyboard to your device and learn very quickly.
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
Flowkey - The easiest way to learn piano with your iPhone or iPad
music21 - Music21 is a Python-based toolkit for computer-aided musicology.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
DaCapo Editor - DaCapo Editor is a free Open Source music sheet editor which allows to easily create nice looking partitions. It is available as a web application as well as a desktop application for Windows, MacOS and Linux.
Synthesia - A fun way to learn how to play the piano.