A useful overview of the Library of Congress classification system for music used at Logan Library
• African American Sheet Music, 1820-1920
• World War I Sheet Music
• Yiddish American Sheet Music
This collection includes digitized scores from the McLellan Lincoln Collection at the John Hay Library; these pieces were written between 1859 and 1923.
The Library of Congress's "American Memory" project includes several collections of nineteenth and early twentieth century American sheet music.
The Sheet Music Consortium includes the digitized sheet music collections of over 20 libraries around the world.
The search interface for the music collection in the University of Rochester (Eastman School of Music) digital repository. Many of the items are unique to the Sibley Library at the Eastman School of Music.
The Petrucci Music Library, also known as IMSLP (International Music Score Library Project), includes over 55,000 works.
This site includes 150 years of Canadian sheet music, courtesy the Library and Archives Canada. It "includes over 20,000 patriotic and parlour songs, piano pieces, sacred music and novelty numbers, some dating back to the 1700s," with both Canadian imprints/publishers and music on Canadian topics.
The Irish Sheet Music Archives website is the home of the online sheet music collections found in the Ward Irish Music Archives located in Milwaukee, Wisconsin.
International Music Score Library Project (IMSLP) - a virtual library of public domain music scores. It contains the complete works of Johann Sebastian Bach in the Bach-Gesellschaft Ausgabe (1851–99) Besides J.S. Bach's complete public domain works, all public domain works of Ludwig van Beethoven, Johannes Brahms, Joseph Canteloube, Emmanuel Chabrier, Ernest Chausson, Frédéric Chopin, Arcangelo Corelli, Claude Debussy, Vincent d'Indy, Paul Dukas, Gabriel Fauré, Pierre-Octave Ferroud, George Frideric Handel, Jean Huré, Albéric Magnard, Giovanni Pierluigi da Palestrina, Maurice Ravel, Albert Roussel, Erik Satie, Florent Schmitt, Franz Schubert, Robert Schumann, Alexander Scriabin and Jean Sibelius are available as well as a large percentage of Wolfgang Amadeus Mozart, Franz Liszt, and the works of many others as well.
Sibley Music Library (Eastman School of Music) Digital Scores
Indiana University Variations Project
Choral Public Domain Library - Also known as CPDL and ChoralWiki. Free choral and solo vocal sheet music