Online scores and indexes

Online scores

Comprehensive collections

• Classical Scores Library
• IMSLP / Petrucci Music Library
• Internet Archive
• Eastman / Sibley Music Library
• Indiana University
• Musopen
• Europeana
• Music Treasures Consortium[details]
  The Music Treasures Consortium is an alliance of libraries with holdings of unique music resources, including music manuscripts, prints, and first and early editions. The consortium includes The British Library, Harvard University, Juilliard School, The Library of Congress, The Morgan Library and Museum, and The New York Public Library.
• Munich Digitization Center (MDZ) of the Bavarian State Library
• The Danish National Digital Sheet Music Archive
  (National Library of Denmark and Copenhagen University Library)
• Harvard Digital Scores Resources [details]
  Authoritative, up-to-date source for worldwide links to non-commercial Web sites offering digitized scores and sheet music, maintained by the Loeb Music Library, Harvard University.
• Online Digital Musical Document Libraries (Wikipedia list)

Composer

• Bach
• Beethoven
• Brahms
• Chopin First Editions Online
• Chopin Early Editions
• Handel
• Liszt
• Lully
• MacDowell
• Mendelssohn
• Mozart
• Nielsen
• Pleyel
• Schubert
• Schumann

Genre

• Choral Public Domain Library
• The Classical String Quartet, 1770-1840
• Corpus Mensurabilis Musicae Electronicum
• Orchestral Parts

Time period

• DIAMM: Digital Image Archive of Medieval Manuscripts
• WLSCM: Web Library of Seventeenth-Century Music

Sheet music

• Music Library (McGill University) Sheet Music Collection
• Library and Archives Canada: Sheet Music from Canada's Past
• Sheet Music Consortium

Score indexes

• Fake Book Index
• Hofmeister XIX: Hofmeister Monatsberichte database (1829-1900)
• Index to Printed Music: Collections & Series (IPM)
• Music-in-Print Series
• Orchestral Music Online (Daniels)
• Répertoire International des Sources Musicales (RISM)
• RISM A/II
• Renaissance music MSS on microfilm at McGill
• Renaissance printed music on microfilm at McGill