orangePlayer is a GPL'd web-based frontend for Music Player Daemon, written in PHP with an Ajax interface. It's increasingly feature-compatible with phpMp2, plus a few extra goodies like related artists and Last.FM integration.
There's a demo version of orangePlayer available -- A note of caution: it's running on a fake mpd server, which is a little shaky.
Requires Apache, PHP5, and (obviously) MPD.
The newest version of orangePlayer includes 'Magic' playlists that are dynamically generated based on the currently playing song, the user's Last.FM profile, etc. To use the magic playlists, you need the following extra dependencies: