Photography All Things Photo, search engine for photographs
American Museum of Photography, includes special exhibits, like the daguerrotype collection, a guided tour, and technical information, such as how to perserve photographs.
AV Photo Finder (Altavista), find photos and art images
CalPhotos, nearly 35,000 images of plants, fungi, animals, people, culture, and landscapes and habitats. "May be used freely for non-commercial purposes."
DHD Photo Gallery, free downloadable photographs
Diggit!, image search engine
New York Public Library Picture Collection
The Picture Collection, from Time, Inc., hundreds of thousands of images form Time, Life, etc. Also, historical photographs.
Picturing the Century, a 100 Years of Photography from the National Archives
Research Page