Bach-Gesellschaft Collected Works -- From the IMSLP (International Music Score Library Project)
Digital Brahms -- An online collection of manuscripts, letters, photos, and other resources about Johannes Brahms
Beethoven Haus: The Bonn Digital Archives -- Contains unique music manuscripts, first editions, letters, and photos from the Beethoven Haus library and museum
Chopin Early Editions -- Courtesy of the University of Chicago Libraries, this collection contains over 400 of his manuscripts
Georg Frideric Händel's Werke -- The digitization project of Friedrich Chrysander's edition of Händel's works, from the Münchener DigitalisierungsZentrum Digitale Bibliothek
Händel's Messiah -- The original manuscript, made available online by the British Library
The Musical Works of Franz Liszt -- Features images from the Carl-Alexander editions of 1907-1936, from the Münchener DigitalisierungsZentrum Digitale Bibliothek
Jean Baptiste Lully Collection -- Contains almost 30 rare 17th- and 18th-century scores of operas and ballets by the 17th-century French composer and his sons, from the University of North Texas Digital Libraries
Mendelssohn's Works -- A digital project of the Münchener DigitalisierungsZentrum Digitale Bibliothek, from Julius Rietz's edition, published from 1874-1877 by Breitkpof & Härtel
Carl Nielsen Edition -- Downloadable music scores of his selected works, from the Danish Centre for Music Publication
Robert Schumann's Autographs -- A digital collection of 25 works by Schumann, from the Universitäts und Landesbibliothen Bonn
Franz Schubert's Autographs -- Contains more than 500 manuscripts, letters, and other documents by and about Schubert
Historic American Sheet Music, 1850-1920 -- an archive of over 3,000 pieces of American sheet music, from the Library of Congress.
The Lester S. Levy Sheet Music Collection -- a searchable sheet music database, from Johns Hopkins University.
Operabase -- a powerful search tool allows you to search the details of over 87,000 opera performances.