Posters American Style - patriotic posters, posters that preach, commercial posters, events posters, etc.
US National Archives & Records Administration Exhibit Hall. Some good American history pieces - the Emancipation Proclamation, government drawings, 20th century photographs, the New Deal and the arts, panoramic photography, 1970s Chicago, World War 2 posters, gifts to presidents, and more.
Unseen America. First person photographs by low income people. Via Bread and Roses, a cultural resource for the labour movement in the New York area.