IMSLP (International Music Score Library Project): A well-known platform to find digitized public domain scores as well as more contemporary pieces by composers who wish to share their material under Creative Commons licenses.
Music for the American Nation: 47,000 pieces of sheet music registered for copyright during the years 1870 to 1885.
African American Sheet Music: This is one of the largest collections of sheet music in any library in the United States. The sheet music, primarily vocal music of American imprint, dates from the 18th century to the present day, with the largest concentration of titles in the period 1840-1950.
Sibley Music Library. Eastman School of Music: The largest academic music library in North America
Ashford Sheet Music Collection: The collection largely contains music from and about Washington State and the Pacific Northwest.
Templeton Music Collection: The sheet music collection of almost 22,000 pieces includes popular tunes dating as far back as 1865.
Chopin Early Editions: The Chopin collection at the University of Chicago Library includes over 400 first and early printed editions of musical compositions by Frédéric Chopin, maintained in the Special Collections Research Center.
Juilliard Manuscript Collection: Among the collection’s holdings are the final working manuscript of Beethoven’s Ninth Symphony prepared for the printer, with extensive revisions, corrections, and alterations by the composer; the autograph manuscript of the final scene of Mozart’s The Marriage of Figaro; and an extensively worked autograph manuscript of the last 50 or so bars of the first movement of Mahler’s Ninth Symphony. The Collection contains 138 items in Special Collections.
Levy Sheet Music Collection: 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 its throrough documentation of nineteenth-century America through popular music.
Guitar Foundation of America :Today the GFA is the largest multinational guitar organization; providing serious guitarists worldwide a full range of educational, literary and performance resources and opportunities.
ChoralWiki: This is the home of the Choral Public Domain Library. Begun in December 1998, CPDL is one of the world's largest free sheet music sites.You can use CPDL to find scores, texts, translations, and information about composers.
Early Women Composers: This page describes some woman who composed music.
Jazz Online:This website/blogzine is geared towards jazz listeners or those who want to learn more about jazz. It has interviews, reviews and articles about jazz musicians. It also includes videos and resources for those people who are interested in Jazz.
Historic American Sheet Music Collection: Duke University's collection includes Negro spirituals, Civil War songs, vaudeville and musical theater numbers and much more.
Library of Congress: the library has compiled links to dozens of performing arts resources.
Project Gutenberg Sheet Music: Project Gutenberg is now expanding their scope. They are now digitizing Public Domain sheet music, including scores. You can download the music.
HathiTrust: HathiTrust is a digital library of thousands of books and other materials including musical scores.