EarMaster - Professional software for ear training and practical music theory on Windows and Mac.
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.
LenMus - LenMus is a free program for learning music.
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.