Notation Player 3 - Score-style MIDI player
NtEd - Music Score Notation editor.
AudioSauna - AudioSauna is a full-featured audio workstation for making music online.
music21 - Music21 is a Python-based toolkit for computer-aided musicology.
MuseScore.org - Create, play back and print beautiful sheet music with free and easy to use music notation software MuseScore. For Windows, Mac and Linux.
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.