A history of photography from its beginnings to the 1920's, by Robert Leggat. Very useful, lots of important basic information.
Prints and Photographs Division of the Library of Congress (main site)
George Eastman House Rochester, NY museum and repository, with a great online collection of images
National Archives and Records Administration online database. Search here for digital copies of FSA and HABS photographs.
Timeline from Still Journal online
Masters of Photography on individual artists
The Photography Museum an entirely online museum, run by an amateur historian, quite worth a visit
Entertaining Optical devices of all sorts
The Camera obscura, Aristotle to Zahn
The Magic Mirror of Life: a search for the Camera Obscura
An essay on the artist St. Memin and his use of the physiognotrace
The Roving Artist's Virtual Silhouette Parlour by a modern silhouette artist
Niepce's Heliograph, the world's first successful photograph, online at University of Texas
The Daguerreian Society a terrific source for all things Daguerreian
Library of Congress' Daguerreotype Collection
Daguerreotype cameras good pictures of the equipment needed to make a daguerreotype
Daguerrian broadside collection of various types of Daguerreotypes
Fox Talbot Museum devoted to the inventor of the calotype (and the first negative/positive process), William Henry Fox Talbot, and located at his home, Lacock Abbey, in Britain.
A good description of the calotype process
Wet collodion process video clips from the Getty Museum
Scully and Osterman, modern wet-plate collodion photographers, with lots of images, reference material and links
Albumen photographs history, science, and preservation
Lost and Found, Rediscovering early photographic processes
Civil War Photographs at the Library of Congress
Panoramic Photographs at the Library of Congress
Lewis Carroll, photographer includes some of his racier pictures of his child friends
Memento mori, a site about post-mortem photography
William Mumler's Spirit Photography from the Museum of Hoaxes
Stereoscopic visions of war and empire, a good visual and textual essay on late 19th/early 20th century American imperialism
Johnson Shawn Museum, in Meadville, PA, home of the Keystone View Company. Dedicated to the history of that company
National Stereoscopic Association
Carleton Watkins at the National Gallery of Art
Matthew Brady's Portraits from the National Portrait Gallery
Francis Frith's photos of Egypt
The Vital Message, by Arthur Conan Doyle (see Appendix 3, On Spirit Photography)
The Silver Sunbeam; a practical and theoretical textbook of sun drawing and photographic printing... by J. Towler, MD (New York: Joseph Ladd, 1864)
The House of Seven Gables by Nathaniel Hawthorne, link to e-text, searchable text, and downloadable files. Novel features a daguerreotypist and daguerreotypes are a motif of the story about the past haunting the present...
Without Sanctuary: Lynching Photography in America (Warning: very disturbing content)
A story on Emory University's handling of the lynching photographs collected by James Allen in the exhibition "Without Sanctuary"
Detroit Publishing Company at the Library of Congress
Smog a collective of contemporary artists and photographers, including Witkin
The Wizard of Photography, the story of George Eastman
The complete history of the discovery of cinematography includes much information on photography, naturally. Site has some technical difficulties due to heavy traffic.
A site on the history of the moving image, with lots of good basic information on technologies
Modern day wet-plate collodion photography by William Dunniway, photographer of historic reenactors
The Big Camera Museum of Photography, a museum in Australia devoted to photography and shaped like a giant 35mm camera