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.

MMA - “MMA—Musical MIDI Accompaniment” is an accompaniment generator.

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.

JJazzLab - Fed up with boring backing tracks ? Try JJazzLab!