I'm just impressed with these two:
last.fm and
amaroK.
Both work perfectly well together.
It's so simple, it's revolutionary. Just listen to your music with your computer's music player - as you do every day - and Last.fm will "scrobble" your playlists. Your tastes, insights and preferences help fuel a worldwide social musical revolution. And it's all free. Sound too good to be true? It's not.
Amarok is a music player for Linux and Unix with an intuitive interface. Amarok makes playing the music you love easier than ever before - and looks good doing it.