Duke University Digital Scriptorium: Historic Amer - http://scriptorium.lib.duke.edu/sheetmusic
This Web site includes 3000 pieces of sheet music from 1850-1920.
Hi-Fido - http://eos.lib.uchicago.edu/html/jazz.html
From the Chicago Jazz Archive at the University of Chicago, Hi-Fido offers sheet music and recordings that date from jazz's beginnings in the early 1910s through Chicago Jazz's heyday in the 1920s up to the Great Depression.
Inventions of Note Sheet Music Collection - http://libraries.mit.edu/music/sheetmusic/index.html
From MIT, this Web site contains old and new technologies as revealed through cover art and song texts from 1890-1920.
Lester S. Levy Sheet Music Collection (Johns Hopki - http://levysheetmusic.mse.jhu.edu/
Popular American music from 1780-1960; includes cover and score images.
Music for the Nation: American Sheet Music 1870-18 - http://memory.loc.gov/ammem/smhtml/smhome.html
Online image collection of over 20,000 songs at the Library of Congress.
Sheet Music Collections Online - http://library.duke.edu/music/sheetmusic/collections.html
Libraries with sheet music collections, many with online searching and/or viewing options.
Sheet Music Consortium - http://digital.library.ucla.edu/sheetmusic/
A group of libraries with the goal of creating an open collection of their digitized sheet music. Many of the items are fully available online, and date from the 18th through the 20th centuries. Member libraries include the Library of Congress, Duke University, the UCLA Music Library, and the National Library of Australia.