The US Library of Congress has a Local History and Genealogy Reading Room in the Thomas Jefferson Building featuring more than 50,000 genealogies and 100,000 local histories among its many collections. According to their post discussing this, “Growing a Family Tree,” the strength in their collections lies in early US history, North American history, and British Isles and German sources.
These international strengths are further supported and enriched by the Library’s royalty, nobility and heraldry collection, making it one of only a few libraries in America that offer such resources. In addition, the vertical files in the reading room contain miscellaneous materials relating to specific family names; to the states, towns, and cities of the U.S.; and to genealogical research in general.
Next trip to Washington, DC, must include this!
Until then, consider their Local History and Genealogy Reading Room (Humanities and Social Sciences Division, Library of Congress) online site as it features a list of The Collections, American Memory collection of digitized materials on US History, and other online and Internet resources.