The International Music Score Library Project provides access to over 200,000 music scores from both the public domain and from composers who are willing to share their music without charge. Over 20,000 audio recordings are also available for certain titles and composers on the site. New scores and recordings are added every month.
The Lester S. Levy Collection of Sheet Music consists of over 29,000 pieces of American popular music. The collection spans the years 1780 to 1980, but its strength is thorough documentation of nineteenth-century America through popular music.
This site contains more than 47,000 pieces of sheet music registered for copyright thanks to the Library of Congress. It includes popular songs, piano music, sacred and secular choral music, solo instrumental music, method books, instructional materials, and music for bands and orchestra.
Musopen is a nonprofit charity that operates out of Palo Alto, California. They are focused on improving access and exposure to music by creating free resources. The site has both free sheet music and recordings available, without copyright restrictions.
A great site that offers scores of classical music. Resources include works by such composers as Bach, Beethoven, Chopin, Handel, Mozart, and many others. All sheet music on this site is in the public domain and free to download, print out, perform, and distribute. Some audio records are included on the site. As of this writing, there are 1,736 pieces of music available on the site and additions are continually being made.
A group of libraries whose goal is to create an open collection of digitized sheet music using the Open Archives Initiative Protocol for Metadata Harvesting. Libraries apart of the consortium include the Library of Congress, National Library of Australia, Duke University, Indiana University, John Hopkins University, UCLA, and University of Maine. The University of California in Los Angeles hosts the consortium. At the latest count, the site provides access to 22 sheet music collections and about 226,914 items. Sheet music is browsable by repository, title, composer, publisher, and date.