America's First Look into the Camera: Daguerreotype Portraits and Views, 1839 - 1864 Over 750 photos with the majority by Mathew Brady. (NOTE: this site may load slowly.)
Art of the American Snapshot An exhibit from the National Gallery of Art, features examples of one hundred years of amateur photography.
Exploring Photography: Photographic Processes Explanations and examples of photographic processes - from the Victoria and Albert Museum in London.