Library of Congress: Digital Collections. Millions of digital items (documents, photographs, maps, films, and audio recordings) from hundreds of collections. New feature, Newspaper Navigator searches 1.56 million historic newspaper photos.
Behold. Use Behold to search over one million Flickr-hosted images that are available free of charge.
Can Stock Photo. Offers over 20 million professional royalty free stock photos.
Getty Open Content Program. The Getty makes available, without charge, all available digital images to which the Getty holds the rights or that are in the public domain to be used for any purpose. No permission is required.
LIFE Magazine Photo Archive. Search millions of photographs from the LIFE photo archive, stretching from the 1750s to today. Most were never published and are now available for the first time through the joint work of LIFE and Google.
Pixabay. From the Pixabay website: "All images and videos on Pixabay are released free of copyrights under Creative Commons CC0. You may download, modify, distribute, and use them royalty free for anything you like, even in commercial applications. Attribution is not required."
Smithsonian Institution. Search over 9.3 million catalog records of museum objects, and library & archives materials. More than 1.3 of these records contain online images, video and sound files, electronic journals and other online resources.
New York Public Library Digital Gallery. Explore one million items digitized from The New York Public Library's collections. This site is a living database with new materials added every day, featuring prints, photographs, maps, manuscripts, streaming video, and more.
Unsplash. All images and videos on Pixabay are released free of copyrights under Creative Commons CC0. You may download, modify, distribute, and use them royalty free for anything you like, even in commercial applications. Attribution is not required.