musicbrowser
I've been using musicbrowser for about a year now at work. The machine on my desk is called "mordor", and runs Ubuntu so naturally I've got a web server running with PHP (and mysql though not needed for this project). I keep all of my mp3s in a single folder on another disk, but I wanted to break them up into genres (this shot shows the medieval music I have at work) so I created a special directory for just symbolic links to various folders. Using symlinks I could arrange the music however I wanted in musicbrowser without actually having to file things any differently. Using this method I can have music show up in different genres as well by just copying the symlink to another genre folder.
see http://musicbrowser.sourceforge.net