USAFacts - Non-partisan, not-for-profit organization that uses publicly available data to create a portrait of America (population, government's finances, government's impact on society).
Library of Congress - Largest library in the world; historical and primary source documents including books, photographs, recordings, maps, newspapers
National Archives - U.S. National Archives and Records Administration (NARA); information on history, genealogy, and the U.S. population and government.