Digital Scores come in all kinds of forms. All of the music on the sites should be in the public domain music (for information about music that is in the public domain you can visit Public Domain Info). Most of the time the music is in pdf format and easily printed and/or downloaded
'Standard Rep' websites
Choral Public Domain Library - Choral sheet music that has mostly been re-typeset
The International Music Score Library Project (IMSLP) - Tens of thousands musical scores
Many universities digitize fragile public domain materials and provide access to them through their library catalogs
Eastman School of Music's Sibley Music Library
Duke University String Quartets (1770-1840)
Composer Specific Sites
Chopin Early Editions at the University of Chicago
Neue Mozart-Ausgabe - Yes, really... the new edition
Manuscripts
Digital Image Archive of Medieval Music - is a leading resource for the study of medieval manuscripts. We present images and metadata for thousands of manuscripts on this website. We also provide a home for scholarly resources and editions, undertake digital restoration of damaged manuscripts and documents, publish high-quality facsimiles, and offer our expertise as consultants.
Popular Sheet Music
African American Sheet Music at Brown University
Lester S Levy Collection at Johns Hopkins University
The Library of Congress Music Division has digitized huge volumes of American music, as well as manuscripts and letters of American-based composers
Wisconsin Sheet Music Database
There are many more such sites listed at the UW-Milwaukee Music Collection website.