One client I wrote: http://github.com/jrockway/bigband

It collects data on which parts of which songs you listen to, and then presents the data as a web interface. Since the data collection and web stuff are async, the web interface can update live as you are listening to music. (The use of this is to see which songs you listen to all the way through, and which ones you ditch at a certain point. For me anyway, the graphs are quite interesting.)