Gnoosic - Even if you don't know what you are looking for - gnod will find it.
Musicroamer - A visual Pandora for discovering new music
Music Walker - Explore and play (via YouTube) songs similar to a seed song.
LivePlasma - Discovery engine map for books, music, and movies
Songstack - Swipe to discover new music for Spotify
Tempso - Finally a place for classical music online!