EarMaster - Professional software for ear training and practical music theory on Windows and Mac.
NtEd - Music Score Notation editor.
Meludia - Listening exercises and to sharpen your ear and music theory skills
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
GNU Solfege - GNU Solfege is free music education software.
music21 - Music21 is a Python-based toolkit for computer-aided musicology.