A continually updated list of interesting music-related digital collections from other libraries, archives, museums and organizations.
Here is a big list of digital music collections that include public domain sheet music, streaming media, manuscripts, and other historical documents. Most are freely accessible.
The Library of Congress has a large number of music-related digital collections.
John Cage Unbound: a "living archive" from the New York Public Library with videos, manuscript excerpts, photographs and more. Check out some other NYPL Digital Exhibits while you're there.
The Association for Cultural Equity is a great place to explore Alan Lomax's field recordings that document historical American music traditions.
Musical Instruments by Darcy Kuronen is an iBook enhanced with forty-eight audio and video clips of expert musicians performing on rare and historical instruments, this e-book edition of MFA Highlights: Musical Instruments brings the world-renowned collection at the Museum of Fine Arts, Boston, to life.