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