Music
Musical Scores and Sheet Music
International Music Score Library Project http://imslp.org/
US Library of Congress collection of sheet music in America http://memory.loc.gov/ammem/smhtml/smhome.html
University of Indiana: http://www.dlib.indiana.edu/variations/scores/
Encoding Musical Scores
Music Encoding Initiative - http://music-encoding.org/home
Music XML - http://www.recordare.com/musicxml (Music XML - Sample - http://en.wikipedia.org/wiki/MusicXML)
TEI with Music http://www.tei-c.org/SIG/Music/twm/index.html
Music Libraries and Archives
Mozart's Letters letters.mozartways.com
NY philharmonic archives https://nyphil.org/history/archives-collections/overview
NYU American Institute for Verdi Studies - http://www.nyu.edu/projects/verdi/index.html
Online Chopin Variorum Edition: http://www.ocve.org.uk/
Carl Maria von Weber Gesamtausgabe (includes TEI encoded documents) http://www.weber-gesamtausgabe.de/de/Index