Here is a website called 250 plus Killer Digital Libraries and Archives from OEDB (Online Education Database). The list contains over 250 libraries and archives that focus mainly on localized, regional, and U.S. history. The contents of digital collections selected are mainly open access, which means general public can access it. The links were made to the root source of digital collections. for instance, if you're looking for the American Memory Project, which was created and housed at the Library of Congress, then you'll find the link for the Library of Congress rather than the link for American Memory although the American Memory Project link was included in the description of the Library of Congress listing.
This list was probably not most updated, but it's nice to have a place to look.