Some online scores can be found in OneSearch. These link to the Music Online database.
The Music Online database includes some in-copyright material from Boosey and Hawkes and selected material from the University Music Editions microfilm series. The collection includes works spanning time periods from the Renaissance to the 21st century. Coverage of score types range from full scores, study scores, piano and vocal scores, and piano reductions.
Of course, no Music Library, or music database, is comprehensive, so below are some important online music score sources for scores that you won't find through OneSearch.
J.S. Bach by Elias Gottlob Haussmann [public domain], from Wikimedia Commons
Beethoven by Joseph Karl Stieler [public domain], from Wikimedia Commons
Hector Berlioz by Franck [public domain], via Wikimedia Commons
Johannes Brahms by Anonymous [public domain], via Wikimedia Commons
Aaron Copland by Cliff1066, via Flikr licensed under Creative Commons
Chopin by Ary Scheffer [public domain], from Wikimedia Commons
Handel by Thomas Hudson [public domain], via Wikimedia Commons
Liszt (Franz Hanfstaengi) [public domain], via Wikimedia Commons
J.B. Lully by Anonymous [public domain], via Wikimedia Commons
Mendelssohn by Eduard Magnus [public domain], from Wikimedia Commons
W.A. Mozart by Anonymous [public domain], via Wikimedia Commons
Carl Nielsen by Anonymous [public Eduard Magnusdomain], via Wikimedia Commons
I. Pleyel by Anonymous [public domain], via Wikimedia Commons
Arnold Schoenberg by Florenka Homolka [public domain], via Wikimedia Commons
Franz Schubert by Wilhelm August Rieder (1796–1880) (Original is in Historisches Museum der Stadt Wien) [Public domain], via Wikimedia Commons
Robert Schumann by Joseph Kriehuber [Public domain], via Wikimedia Commons
Johann Strauss by Anonymous [public domain], via Wikimedia Commons
Carl Maria Von Weber by Anonymous [public domain], via Wikimedia Commons
Except for logos, Canva designs, AI generated images or where otherwise indicated, content in this guide is licensed under a Creative Commons Attribution-ShareAlike 4.0 International Licence.