International Music Score Library Project - includes hundreds of thousands of public domain scores, including those of a variety of classical composers
MusOpen - a non-profit organization providing access to public domain scores, including those of a variety of classical composers
Sheet Music from Canada's Past - from Library and Archives Canada and including over 20,000 pieces of music published in Canada from the 1900s to 1920
Historic American Sheet Music: 1850-1920 - from Duke University Libraries and including over 3,000 pieces of music published in the United States between 1850 and 1920
Music for the Nation: American Sheet Music 1820-1885 – from the United States Library of Congress and including over 47,000 music pieces published in the United States from 1870 to 1885
The University of Washington Libraries - Musical Scores – including music pieces from the 1600s to the 1800s