NtEd - Music Score Notation editor.
Notation Player 3 - Score-style MIDI player
DaCapo Editor - DaCapo Editor is a free Open Source music sheet editor which allows to easily create nice looking partitions. It is available as a web application as well as a desktop application for Windows, MacOS and Linux.
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.