allmusic - An outstanding, free resource for finding information on all genres of music. It is particularly useful for finding information on world music (classifed as International), jazz, blues, and popular music. You can browse selected categories or search for specific terms. Create an account to curate your own lists and rank albums.
J-DISC - An online discography of jazz created by the Center for Jazz Studies at Columbia University. It currently documents the complete recorded history of 16 jazz artists, with an additional 45 planned for inclusion.
WorldCat - The free version of WorldCat.
IMSLP - Access to online, public domain scores.