dudl is a collection of tools to grab, encode, and rename MP3 files, archive them, and maintain a database of all files. On top of this runs the dudld Jukebox daemon that plays your files through gstreamer. dudld's strengths are the powerful tag based filters, the capability to cope with very large collections, and the network protocol to control its operation. There is a full featured command-line client, a minimalistic GTK client, and a lirc client. Writing new clients should be easy with the supplied client libraries.
Theory is a Python/Pylons-based Web client for the MPD system that uses Ajax where it makes sense. It includes drag/drop playlist reordering, Amazon cover art retrieval, a full-screen "party" mode, lyrics fetching from lyricswiki.org, automatic linking to the artist's Wikipedia page, and a decent theme.