MuseScore is an open source and free music notation software. For support, contribution, bug reports, visit MuseScore.org. Fork and make pull requests!

Tomahawk, the multi-source music player

cursynth is a polyphonic, MIDI enabled, subtractive synth that runs in your
terminal with an ascii interface.

cursynth will connect to all MIDI devices and send output to the default audio
device on your system.

Collaborative Programmable Music

Web-based music player for the cloud

MusicBrainz Java data bindings using Hibernate

These libraries for Python, PHP, Ruby, Node.js and Java implement the subset of OAuth that is required to talk to the Rdio service with minimal external dependencies.

A JavaScript Sound API supporting MP3, MPEG4 and HTML5 audio + RTMP, providing reliable cross-browser/platform audio control in as little as 12 KB. BSD licensed.

A Javascript library for working with Audio. It provides a consistent API for loading and playing audio on different browsers and devices. Currently supports WebAudio, HTML5 Audio, Cordova / PhoneGap, and a Flash fallback.

Echo Nest Remix is the Internet Synthesizer. Make amazing things from music, automatically.
Turn any music or video into Python or JavaScript code.

This is an open and public initiative to bring more harmony into HTML5 Audio implementations. The specifications are missing some features and sometime leaves room for interpretation. We want to un-mute the Web and make Audio rock!

Making life easy to create a MIDI-app on the web. Includes a library to program synesthesia into your app for memory recognition or for creating trippy effects. Convert soundfonts for Guitar, Bass, Drums, ect. into code that can be read by the browser. Supports multiple simultaneous instruments and perfect timing.

A free and open platform giving all musicians access to tools that let them manage, promote, and sell their music online.

music library manager and MusicBrainz tagger

MusicBrainz Picard is a cross-platform (Linux/Mac OS X/Windows) application written in Python and is the official MusicBrainz tagger.

Picard supports the majority of audio file formats, is capable of using audio fingerprints (AcoustIDs), performing CD lookups and disc ID submissions, and it has excellent Unicode support.

Mopidy is a music server which can play music both from multiple sources, like your local hard drive, radio streams, and from Spotify and SoundCloud. Searches combines results from all music sources, and you can mix tracks from all sources in your play queue. Your playlists from Spotify or SoundCloud are also available for use.

Play is an employee-powered iTunes-based client-driven distributed music server for your office.

The Live Coding Synth for Everyone

Open-source audio synthesis, processing, & analysis platform.