Software modular synth
LilyPond sheet music text editor
A Fasttracker II compatible music editor
ossia score, an interactive sequencer for the intermedia arts
fl_toadflax is a viewer/editor for abc music notation files
Free music notation & composition software
Featureful native Commodore 64 music tracker with MIDI/XM converter
Text-based Music Programming Tools
Give the chord progression and a style and jchordbox generates a song.
A set of scripts to manipulate audio tags of various audio formats.