Are you looking for accessible collections of musical scores? Take a look at these resources:
Set up as a "wiki" style page, you can search for public domain music scores, or browse by composer, time peried or instrument.
According to the site, "The scores and libretti in this Virtual Collection include first and early editions and manuscript copies of music from the eighteenth and early nineteenth centuries by J.S. Bach and Bach family members, Mozart, Schubert and other composers, as well as multiple versions of nineteenth century opera scores, seminal works of musical modernism, and music of the Second Viennese School."
Over 140 unique manuscripts, with original markings, changes and sketches also included.
For more links to online sheet music, visit our Performing Arts Library Research Guide.