Social and Historical Foundations of American Music

Resource NYPL archivist Matt Snyder mentioned.

Incredible collection -- quite a few digitized items too!

German national archives.

Many divisions, includes archival collections and more.

Ever changing.

Digital Library of Written Cultural Heritage

Not archives per se, but a searchable catalog of early music imprints, and music manuscripts written after 1600.

Listing of frequently consulted archival repositories and collections (from a rock/pop/blues perspective).

Libraries, Archives, and Research Centers for American Music