AP Images - Photographs from as early as the 1840's and other graphics spanning over 160 years of history.
American Memory (Library of Congress) "provides free and open access through the Internet to written and spoken words, sound recordings, still and moving images, prints, maps, and sheet music that document the American experience."
Leonard Carpenter Panama Canal Collection - "primarily consists of photographs of Panama and the Panama Canal Zone during construction, beginning in 1914."
LIFE Photo Archive - "Search millions of photographs from the LIFE photo archive, stretching from the 1750s to today."
Time & Life Pictures - "includes some of the greatest photographers of the 20th century, such as Alfred Eisenstaedt, Margaret Bourke-White, Andreas Feininger, John Dominis, Nina Leen and Gjon Mili, whose photographs have adorned the pages of Time, Life and other Time Inc. publications."