NtEd - Music Score Notation editor.
Virtual MIDI Piano Keyboard - Virtual MIDI Piano Keyboard is a MIDI events generator and receiver.
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
EveryonePiano - Everyone Piano is the best free computer keyboard piano software, which supports downloading 3 types of music score formats like stave, right and left hand numbered musical notation and EOP file.
music21 - Music21 is a Python-based toolkit for computer-aided musicology.
Synthesia - A fun way to learn how to play the piano.