- MuseScore, a musical notation app
- Open source, free. Supports input via MIDI.
- Sonic Pi, a program that allows you to write music
with Ruby-style code
- Furnace, a chiptune maker.
(HN Comments)
- Mixxx, an open source DJ app. Found this from Hacker
News.
- Spotifyd, a Spotify client running as
a UNIX daemon (HN)