Khövsgöl is a minimal, highly configurable music player and browser. It can easily handle very large collections of music. It's designed for the GNOME desktop, but can run well in other environments. It works fine as a standalone player, but can also be used to remotely control music daemons such as MPD.
Guayadeque is a music player that aims to be easy to use and fast even for huge music collections. It allows you to automatically or manually download covers, easily organize the music library, edit the ratings of the tracks, add labels to the artists, albums, and tracks, listen to shoutcast radios, download lyrics, show artist information based on last.fm content, and more. It includes a simple, built-in tag editor with picture support.
Cuberok is a music player and collection manager with a lightweight interface. It supports music collections, CUE sheets, Shoutcast/icecast streams, a Last.FM and Libre.FM scrobbler, downloading images for albums and artists from Last.FM, information about artists from Last.FM, and lyrics from LyricWiki.org, automatic ratings for songs, and support for many playlists at one time.
Impro-Visor is a music notation and playback tool for helping jazz musicians learn to improvise. It features a notation GUI, automated playback of chords and rhythm using MIDI, and improvisation advice provided in a variety of ways, including being able to improvise jazz itself. Data are stored as open-format text files. MIDI and MusicXML export are also available.