Captain Chords - Write chord progressions in your favorite DAW
NtEd - Music Score Notation editor.
Odesi - ODESI is a music sketchpad that exports your projects to Audio and MIDI.
music21 - Music21 is a Python-based toolkit for computer-aided musicology.
Cthulhu - VST MIDI-FX plugin for harmonic inspiration
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.