Gnoosic - Even if you don't know what you are looking for - gnod will find it.
Musicroamer - A visual Pandora for discovering new music
stats.fm - With the click of a button you'll be logged with your Spotify account and you'll instantly gain access to a valhalla of cool stats and insights.
LivePlasma - Discovery engine map for books, music, and movies
Spotify - Map shows when two people play same song at same time
Music Walker - Explore and play (via YouTube) songs similar to a seed song.