How about a Spotify client?

[QuiteTrite]

Of course, it would require being able to use facebook authentication (just in case people set their spotify up that way), and should save any downloaded music to the Music share so it can be added into the iTunes and Plex services.

[MikeG.6.5]

Doesn't Spotify use DRM on the music?

So adding to Plex or iTunes is a no go it that's the case...