Most of the first-generation mashups have been limited to the data freely available from governments, sites with APIs etc.; I can't wait to see what we'll get now that they can use Freebase. I can't wait to use it myself, for that matter. Best of all, those magical letters: "CC-BY".
This has everything I want in a music site. It's just about perfect. Tags, so albums are well-classified and easy to find more of; preview or full play on an in-browser player (including the ability to play all the albums on a page of search results -- instant radio); downloads via BitTorrent in either MP3 or Ogg Vorbis; comments; search by license/restrictions; a huge variety of RSS feeds to keep up with new music... And best of all, more than half of the dozen or so albums I listened to were worth actually having. After five minutes I'm in love.