Searchable by type of music, this archival collection of sheet music is a full-text electronic resource. The collection consists of over 29,000 pieces of American popular music spanning two hundred years from 1780 to 1980
Digital Scriptorium is a growing consortium of American libraries and museums committed to free online access to their collections of pre-modern manuscripts. Here you can find many digitized copies of medieval texts including sheet music for musical chants.
The Library of Congress has an extensive online collection of primary source material. Their online collection includes prints, photographs, historic newspapers, maps, sound recordings, oral histories, diaries, letters, family papers of prominent Americans, manuscripts, and more.