U.S. Government Graphics and Photos - most of the images and graphics are available for use in the public domain, and they may be used and reproduced without permission or fee
Smithsonian Institution - digital library - eclectic mix on sources and SIRI Image Gallery
American Memory - digitized collections of oral histories, letters, photographs, documents.
Images of American Political History - over 500 public domain images with browse or search capability
Photo and Image Collections - a growing number of photo and image galleries - some classical and historical photographs taken by USGS scientists.
New York Public Library - a collection of 30,000 digitized images from books, magazines and newspapers as well as original photographs, prints and postcards, mostly created before 1923.
UNESCO's photobank - over 10,000 photos on the Organization’s activities: education, science, culture and communication