AMDOCS: Documents for the Study of American History - links arranged chronologically to a variety of primary sources in many different sites.
United States. Library of Congress. American Memory - "written and spoken words, sound recordings, still and moving images, prints, maps, and sheet music that document the American experience."
Early Americas Digital Archive - "collection of electronic texts and links to texts originally written in or about the Americas from 1492 to approximately 1820."
Making of America (MoA) - "primary sources in American social history from the antebellum period through reconstruction." Collaborative project of the University of Michigan and Cornell University.
Presidential Timeline of the Twentieth Century: timeline created with links to digital content - documents, audio, video - from 12 U.S. Presidential Libraries.
Toolbox Library: Primary Resources in U.S. History and Literature. At the National Humanities Center.
Smithsonian Institution. Archives of American Art. American Art History (primary sources).