The American Museum of Photography features brief views of their exhibits of historic photography.
Archives of American Art: Photography (Smithsonian Institute) can be either searched or browsed. A filter for photography has already been added to this search page, but you can add additional search terms in the box near the top of the form, or you can sort the archival collections using the tabs below the search box. Naturally, you can also just page through the 200 collections. Only some of the collections have been digitized.
The Art Institute of Chicago Photography and Media collection is described on this page, and a small selection of images are offered. The treasure is nearly buried near the bottom of the page, where a link allows you to see all 22,683 artworks which leads you to a randomly ordered view of all the digitized photos in the AIC collection. Click on Show Filters at the top of the page to find tools to help you sort and refine the results list.
The Center for Creative Photography at the University of Arizona is a museum, library, study center and archive containing over 270 archival collections with more than 110,000 works by 2200 well known, 20th century, North American photographers. The Fine Art Photography page.allows you to browse a complete list of artists by name, and view a finding aid to the works in the collection. Unfortunately this finding aid is missing many links to the online collection. Using the much shorter list of artists on the Online collections page is a more direct method for finding images.
George Eastman House is lamed for the founder of the Eastman Kodak company and a pioneer of popular photography. His estate in Rochester, NY has become one of the most significant photography museums, with an archive of photography and cinematography that contains "several million objects." Search the Photography Collection by artist to find a list of photographs in the collection. Those that show a thumbnail have been digitized and can be enlarged.
The International Center of Photography (ICP) offers a browsable list of Artists from their online collections. Each artists' page contains a portrait of the artist, a biographical sketch (when available) and a collection of photographs. Each image can be enlarged and the photo page includes data about the image as well as credit line and copyright information for your citation..
Library of Congress Prints and Photographs Digital Collections can be browsed by collection. Not all of the collections are of photography, but there are notable collections of historical photographs including early photography from the Civil War, techniques such as Daguerreotypes, Stereograph cards and Panoramic photos, as well as collections by Ansel Adams, Mathew Brady and Edward S. Curtis
The Museum of Contemporary Photography at Columbia College has an online collection that can be browsed by artist. Their Midwest Photographers Project also features small portfolios for over 70 artists from the Midwest.
You may also want to consult:
The Clark Art Institute Library has an excellent library guide for rsearch in photography, which may also be of use to you.