What I did a long time ago was that I had a plugin in Winamp that transmitted all the music I played onto a server which I controlled. That server processed the information, got a thumbnail from Amazon.com with web services and uploaded it to this site onto MP3 I Play
To me it was very much an excuse to play around with the technology and familiarise myself with ID3 tagging in Python and the Amazon.com web services using pyAmazon.
With Audioscrobbler they've taken this whole thing seriously, done it much much better and opened it up to the public.