CLIR Hidden Collections Registry
A discovery tool to highlight rare and unique library, archival, and museum collections.

National Union Catalogue of Manuscript Collections (NUCMC)
Catalog records maintained by the Library of Congress.

National Archives and Records Administration (NARA)
U.S. Federal agency responsible for Federal records; includes the Online Public Access Catalog (formerly known as the Archival Research Catalog) for searching holdings nationwide, and Access to Archival Databases (AAD), which allows searches of NARA's most popular electronic records.

Finding Historical Primary Sources (UC Berkeley)

Primary Sources on the Web: Finding, Evaluating, Using
From the History Section of the Reference and User Services Association (RUSA) of the American Library Association (ALA).