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.
MediaZoo is an online personal media library and player that allows users listen to their music and invite close friends to their music room. Its mission is to allow users, musicians, and developers to start collaborating on creating a unique, open, and free music experience in the cloud.
m9u is a music server, similar to mpd or xmms2. It sits in the background, waiting for commands to be delivered via a 9p file system. m9u embraces the Unix philosophy of doing one thing well. As such, it does not try to catalog or otherwise organize music, but aims for the more humble goal of simply playing it. A side effect of this approach is that m9u is trivially extensible to domains aside from music (for example, an image slideshow).
Smitten differs from traditional media centres in three major ways. It contains no media playback at all, which makes it very small (and suitable for porting to embedded systems) and quick to initialize. It targets a TV display and interface, which means there's less information on any particular screen, and a much simpler control interface consisting of left, right, up, down, and select. No mouse is required. It is also highly skinnable, and cross-platform to most systems. Essentially, it provides an easy-to-use menu system that allows the user to navigate a hierarchy of files, and view them using an (external) tool.