As far as I can determine, images from these sites are in the public domain or have been released under the equivalent Creative Commons License 0. Please take time to verify the copyright status of any image you download. I update this page often, so check back from time to time.
Central Intelligence Agency (CIA): Regional and World Maps
Creative Commons (500 million free images, but not all are public domain)
flickr: Biodiversity Heritage Library (240,000+ antique photos)
flickr: Biodiversity Heritage Library Jardin de la Malmaison (Pierre-Joseph Redouté)
flickr: Great Sand Dunes National Park
flickr: John James Audubon Collection from Biodiversity Heritage Library (BHL)
Government of Canada: Reference Maps
Library of Congress: WPA (Work Projects Administration) Posters
NASA Earth Observatory Explorer
NASA Jet Propulsion Laboratory Photojournal
NASA Jet Propulsion Laboratory: Visions of the Future (Vintage-style Space Travel Posters)
National Archives and Records Administration NARA
National Gallery of Art: John James Audubon
New York Public Library: Pierre-Joseph Redouté’s Les liliacées (1805-1816)
New York Public Library Public Domain Collection
New York Public Library: Robert John Thornton’s Temple of Flora
Picryl (5.8 million public domain images)
Picryl: Detroit Publishing Company (6469 photochrom prints)
Picryl: Antique and Vintage Travel Posters
Rawpixel: Francois Levaillant’s Natural History of Parrots (1801-1805)
Rawpixel: John James Audubon’s The Birds of America
Rawpixel: John James Audubon’s The Viviparous Quadrupeds of North America
Rawpixel: Kingdom Animalia (vintage animal images)
Rawpixel: Nile Illustrations by David Roberts (1796-1864)
Rawpixel: Pierre-Joseph Redouté’s Les liliacées (1805)
Rawpixel: Robert John Thornton’s The Temple of Flora
Smithsonian Institution Photos and Images
Smithsonian Libraries Image Gallery
USDA Vintage Pomological (fruit) Watercolor Collection
Wikipedia Commons: Thirty-Six Views of Mount Fuji (46 prints by Hokusai)
From the Public Domain 