jiggyMPD is a Web-based client for the Music Player Daemon and a companion tray icon. It is in relatively early development, but usable now. jiggyMPD extends the MPD data to add features MPD clients typically lack, such as ratings, date added, and last played date. This allows for some unique features. The jiggy data can be queried for smart playlist-type functionality. For example, "added <= 30, rating = 0, playcount <= 2, lastplayed > 1" will tell the client to find all songs added to your collection within 30 days, not yet rated, played 2 or less times, and not played today. It also fetches album art and lyrics.
Beets is a media library management system designed for obsessive-compulsive music geeks. It catalogs your collection, automatically improving its metadata as it goes using the MusicBrainz database. It also downloads cover art for albums it imports. Then it provides a bouquet of tools for manipulating and accessing your music. Beets also includes a music player that speaks the MPD protocol, so you can play music in your beets library using any MPD client.
pyscmpd is a music player daemon designed for use on Raspberry Pis. It implements a subset of the MPD protocol, but instead of serving local MP3 files, it connects to SoundCloud.com and serves the streams found there. It supports a huge number of front-end clients through MPD, including ncmpcpp and Sonata. It allows you to configure which parts of SoundCloud are made available for browsing by defining categories and adding groups or users to them.