Spotify (http://www.spotify.com) just updated itself, and it now plays local MP3s !
This is awesome as I can now shuffle mode all my music in one place 🙂
Update on how this works:
You add 'local files' which are stored only for that spotify install, you can then play from the local list or add to a central playlist.
Playlists only store the track details, not the file location. This means MP3's can be in different folders on each PC and the playlist will still work, the track just has to be in the 'local files' list.
It is not perfect though:
You cannot navigate into local albums or artists, local files is just one big list.
You cannot specify a location to automatically scan for new tracks, manual importing is required.