Music Scores Online

from the Library of Congress: browse a variety of sheet music collections: Civil War band music, 19th century song sheets, etc.. Includes links to other online collections.

operetta and musicals digitized by the University of Illinois

Digital images of 3000+ pieces of sheet music, "published in the United States between 1850 and 1920."

Digitized sheet music from a number of libraries, including UCLA, Indiana University, and Johns Hopkins University (may overlap previously listed collections)

other sites as listed on Harvard's Online Resources for Music Scholars