American Memory -- This Library of Congress collection provides free and open access to a wide range of primary source material documenting American history and culture, including text, sound recordings, still and moving images, prints, maps, and sheet music. Its collections are as follows:
In addition to the American Memory links, other collections include:
Densho.org -- Densho documents the experience of Japanese Americans who were incarcerated during World War II, providing access to digitized internment camp newspapers, firsthand accounts, video interviews, and more (you'll need to sign up for a free account to use this).
History Matters -- This site, created by the American Social History Project/Center for Media and Learning (CUNY) and the Center for History and New Media (George Mason University) contains a wealth of primary source material, research guidance, and annotated Web links.
Life Photo Archive This collaboration with Google provides access to millions of photographs, from the 1750s on, most of which were never published.
Making of America (University of Michigan)
Making of America (Cornell University)
Native American History (University of Washington)
Voice of the Shuttle: Extensive portal of History sites
Not finding what you're looking for? Ask a Librarian!