General:
NYPL Digital Collections Digitized images from the New York Public Library's vast collections.
Creative Commons Search Search for images that are in the public domain OR have been granted creative commons status.
ContentDM Collections: Browse Look through all the digital repositories in the ContentDM system.
Hollis Images | Harvard University A free online image database.
Smithsonian Open Access Art, science, historical images & more that are public domain/free to use.
Art / Architecture / Design:
ArtBabble Like YouTube, but just art-related videos.
Cooper-Hewitt Collection Digitized collection of the Smithsonian's Design museum.
Getty Research Portal Access digitized art history publications, rare books, and related literature. Like Google Books, but just for art stuff.
Metropolitan Museum of Art | Collection Includes access to 300,000 open access images (labeled as "OASC")
Mother of All Art & Art History Links | Online Image Resources A list of image sites, organized geographically.
Visual Arts Data Service (VADS) 100,000 images free for use in education.
Video Data Bank A site committed to video art & alternative media in the U.S.
Post Video Art | Streaming Video Art & Experimental Film View video art streaming on the web.
Bridgeman Art
History:
Ad*Access | Duke University Over 7000 digitized images of advertisements
Digital Collections | Library of Congress All sorts of cool stuff relating to American visual culture.
LIFE photo archive hosted by Google Photographs from the LIFE magazine photo archive, 1750s to today
Science & Technology:
Animal Sound & Video Catalog | Macaulay Library A database of animal sounds & videos.
Classic Illustrated Zoologies & Related Works, 1550-1900 From the NYPL
NASA Images Photos of the universe, solar system, Earth, and more
USDA Pomological Watercolor Collection 7,584 watercolor paintings, lithographs and line drawings of fruits and nuts grown in the United States; all created between 1886 to 1942.
Popular Culture:
