These sites below point to scholarly editions, but some include popular musics.
Multiple collections including 19th century collected works editions for Beethoven, Händel, Liszt, Mendelssohn, Schubert, and Schumann and the Denkmäler deutscher Tonkunst: first and second series. See also Manuscripts for more content.
Music manuscripts, sketches and printed music by Beethoven and other composers.
The Online Chopin Variorum Edition "provides digital images of the manuscripts and first editions of select works by the Polish composer Fryderyk Chopin. Its principal aim is to facilitate and enhance comparative analysis of these primary sources, attaining a degree of flexibility beyond that of extant printed editions of Chopin’s music and indeed of any composer to date. Users are guided in their consultation of the sources by the background information and sample ‘critical commentaries’ provided by the project team. The end result is a new type of ‘musical edition’, one that users themselves construct dynamically and interactively."
"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.” PDF downloads.
IMSLP (International Music Score Library Project.) (Petrucci Music Library)
Free music scores in PDF format contributed by libraries and individuals.
WARNING!---Older editions. Scores could be out-of-date, missing pages and/or contain errors!
“The following selections lead to experimental prototypes of the ways in which musical scores and recording liner notes might be used in conjunction with sound recordings available online through VARIATIONS.” Includes a few examples from the operatic, song, orchestral, choral, solo instrumental, and chamber ensemble literature from the Indiana University collection. PDF downloads
“Here you can find modern editions of a lot of music from the 17:th and 18:th century, many of them not available in any other modern form.” (“Lysator is an academic computer club at Linköping University. Its purpose is to provide members the opportunity to explore computers and computer science.” PDF downloads.
The digitized critical edition.
“Scores and books in the public domain. Many of these are unique to the Sibley Music Library collection.” PDF downloads.
Brown University, John Hay Library. Sheet Music Collection.
“The Sheet Music Collection at the John Hay Library, Brown University, is an outgrowth of the Harris Collection of American Poetry and Plays, and contains approximately 500,000 pieces of sheet music, largely American imprint vocal music dating from the late 18th century to the present day; the bulk of the titles date between 1800 and 1950.”
Created by Mary Kay Duggan. “A virtual library of some 2,700 pieces of sheet music published in California between 1852 and 1900, together with related materials such as a San Francisco publisher's catalog of 1872, programs, songsheets, advertisements, and photographs.”
British Columbia Sheet Music.
"Sheet music about British Columbia, published in British Columbia or composed by British Columbians"
The Frances G. Spencer Collection of American Sheet Music, Baylor University.
"One of the few collections of its kind and size in the country. Acquired by Baylor University in 1965, this collection of approximately 30,000 titles was the life-long pursuit of the avid and well-respected sheet music collector for whom the collection is named.”
A VAST array of music manuscripts, printed music and sheet music collections. Subjects range from classical to popular, patriotic songs, show tunes, Civil War music.....
“A group of libraries working toward the goal of building an open collection of digitized sheet music using the Open Archives Initiative:Protocol for Metadata Harvesting (OAI:PMH).” Sponsored by UCLA.