MUTH 35500: Music History 3

a huge collection of digitized scores online, searchable by composer, title, genre, and more

a partnership of academic research libraries that have digitized much of their material, including scores

Europeana Collections provides access to over 50 million digitized items – books, music, artworks and more.

full scans from public domain scores at Eastman's Sibley Music Library

directory of sheet music of art & popular songs; IPA translations available; edition information usually available.

the scores section from CPDL wiki site -- formats include PDF and Finale versions

All of Mozart's works as published in Bärenreiter (no vocal scores, though).

Schubert scores, manuscripts, and letters, all digitized from collections across the world.