Google Advanced Image Search (select free to use or share under usage rights)
Library of Congress Photo Stream on Flickr
NARA (National Archives and Records Administration)
Flickr: The British Library's Photostream
National Archives Presidential Libraries and Museums
NYPL Digital Gallery (Some of the images in the Digital Gallery may be subject to third party rights such as copyright and/or rights of privacy/publicity)
Smithsonian Flickr PhotoStream
Internet Archive Moving Images
National Parks Service Imagebase
American Memory Timeline for Teachers
New York Public Library Image Archives
Artifact and Analysis (Smithsonian)
Copyright-friendly images and sound Pathfinder