The International Music Score Library Project is a vast collection that comprises both public domain scores and those of contemporary composers who wish to have their work made openly accessible.
Duke University Libraries provides a list of useful online sheet music resources from a wide array of periods and genres.
The Sheet Music Consortium comprises collections from over 20 libraries. Not all of the records are linked to digitized sheet music, but the search results can be limited to show only digitized content.
The Library of Congress Historic Sheet Music Collection, 1800-1922 comprises approximately 9000 items. The collection focuses primarily on American music for piano and voice.