The Library of Congress's Prints and Photographs Division has digitized an impressive array of image formats / types and subjects. Those digital materials, grouped by collection, can be found here. Similarly, the holdings of the National Archives and Records Administration include 3 million digital photographs and graphic images in the Still Picture collections at the National Archives at College Park; those digitized materials can be found by scrolling to the bottom of the page (Photographs and Graphic Works in College Park, Maryland).