I've been somewhat obsessed with last.fm recently. I remember checking the site out a couple of years ago but I don't recall it being so big and diverse. I can plug in any random band and it'll usually figure out some kind of 'related' playlist.
The last.fm API is also a really nifty tool. I've played around with it enough to integrate tracks I've been listening to onto the homepage here, but I can imagine a million better and more complicated uses for the amount of information the API makes available. And you can't beat a RESTful API that will return JSON data -- so easy to use. I'll have to create some kind of charting app for the site, or something.