Any number of organizations, not-for-profit or for-profit, have started to make music scores available online.
Brown University Library: Digital Sheet Music Collections
• African American Sheet Music, 1820-1920
• World War I Sheet Music
• Yiddish American Sheet Music
Brown University Library: Lincoln 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.
Eastman School of Music Digitized Scores
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.
Irish Sheet Music Archives
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.
Library of Congress, Sheet Music Collections
The Library of Congress's Digital Collections includes several nineteenth and early twentieth century American sheet music.
Open Music Library
The Open Music Library is an initiative from Alexander Street to build the world’s largest free index of digital resources for the study of music.
Petrucci Music Library
The Petrucci Music Library, also known as IMSLP (International Music Score Library Project), includes over 55,000 works.
Sheet Music Consortium
The Sheet Music Consortium includes the digitized sheet music collections of over 20 libraries around the world.
Sheet Music from Canada's Past
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.