VLC Media Player - VLC is a free and open source cross-platform multimedia player and framework.
Visual Beat - Interactive music video
Music Player Daemon - Music Player Daemon is a flexible, powerful, server-side application for playing music.
Splice Beat Maker - Make and share beats in your browser
Mopidy - Mopidy is an extensible music server written in Python. Mopidy plays music from local disk, Spotify, SoundCloud, Google Play Music, and more. You edit the playlist from any phone, tablet, or computer using a range of MPD and web clients.
Sampulator - Make (and record) beats on your keyboard