Musicroamer - A visual Pandora for discovering new music
BandNext - Discover bands that sound similar to artists you already love with BandNext. A single click saves your results to a Youtube playlist.
Music Walker - Explore and play (via YouTube) songs similar to a seed song.
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.
Gnoosic - Even if you don't know what you are looking for - gnod will find it.
Spotify - Map shows when two people play same song at same time