The Online Library of Congress

From the the Library of Congress, there are enourmous amounts of online musical documents available in subjects as diverse as African American music, Folk Music, composers, instruments, film music, jazz, sheet music, as well as the Moldenauer Archives, with 3500 items documenting the history of Western Music.

In Jon Newsom's Introduction to the archives, he traces the origin of Hans Moldenauer's bequest and the process that he went through to build a collection of historical source material for future researchers. Newsom writes of Moldenauer,

...he had the connoisseur’s sense of the intrinsic worth of a beautiful and rare page, and so, besides their research value, some
of his manuscripts--and these were the ones he treasured most highly--represent, in his words, "tangible links with the esoteric efforts which produced them in the composer’s hour of inspiration." We stand in awe before the pages on which Mozart or Brahms wrote down a memorable piece and wonder at the mysterious role
that the composer’s original notation plays in the relationship between his musical idea, its realization as sound by a performer, and the listener.

Kudos to the Library of Congress for presenting the information here in such an attractive and easily searchable format.

Warning: you can spend hours looking at these documents...