NtEd - Music Score Notation editor.
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
music21 - Music21 is a Python-based toolkit for computer-aided musicology.
JJazzLab - Fed up with boring backing tracks ? Try JJazzLab!
Guitar Pro 7 - Create, play and share your tabs
ABCexplorer - ABCexplorer is a musical creation utility that comes with advanced support to create, edit, play, convert and manage musical files with ABC text format support.