http://digitalcollections.oscars.org/cdm/
In addition to correspondence, photographs, early release fliers, full issues of rare periodicals, and movie star ephemera, also includes approximately 300 pieces of sheet music from the early twentieth century.
http://digital.library.ucla.edu/apam/
The UCLA Music Library's Archive of Popular American Music is a research collection covering the history of popular music in the United States from 1790 to the present. It has almost 450,000 pieces of sheet music, anthologies, and arrangements for band and orchestra.
http://www.boosey.com/cr/perusals/
Music publisher allows free viewing of many scores online. However, there is no downloading or printing.
http://www2.cpdl.org
Offers public-domain choral and vocal scores, texts, translations. Currently contains almost 18,000 works by some 2,400 composers.
http://www.diamm.ac.uk/
A digital archive of images of European sources of medieval polyphonic music, captured directly from the original document.
http://imslp.org
A virtual library of public domain music scores (i.e., music no longer under copyright protection) as well as music scores from composers willing to share their music without charge. Musical works can be searched by composer name, time period, nationality, or by genre, instrumentation, and language. Also has a limited number of music recordings.
http://vc.lib.harvard.edu/vc/deliver/home?_collection=scores
Scanned images of rare and unique music scores, primarily from the eighteenth and nineteenth centuries, in the Loeb Music Library at Harvard.
http://www.mutopiaproject.org/
Offers sheet music editions of classical music for free download. These are based on editions in the public domain, and include works by Bach, Beethoven, Chopin, Handel, Mozart, and many others. Contains approximately 1,900 pieces of music.
http://digital2.library.ucla.edu/sheetmusic/index.html
The Consortium is comprised of a number of member libraries (including UCLA) that are interested in building "an open collection of digitized sheet music." The website currently offers cross collection searching of 22 sheet music collections, with a total of 226,000 items.
https://urresearch.rochester.edu/viewInstitutionalCollection.action?collectionId=63
A digital collection of over 20,000 public domain scores in the Sibley Library at the Eastman School of Music.