Gnoosic - Even if you don't know what you are looking for - gnod will find it.
MusicBrainz - A community-maintained open source database and encyclopedia of music information.
Musicroamer - A visual Pandora for discovering new music
Discogs - Discover music on Discogs, the largest online music database. Buy and sell music with collectors in the Marketplace.
Songstack - Swipe to discover new music for Spotify
AllMusic - AllMusic.