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