American Memory - http://memory.loc.gov/ammem/
A variety of searchable photo collections from the Library of Congress, including: historic American architecture, baseball cards, women's suffrage, jazz legends, World War II, vaudeville, and more.
American Museum of Photography - http://www.photographymuseum.com/
A "museum without walls," changing exhibits feature masterworks, historical photos, early daguerreotypes and current art photography.
eLibrary - http://www.kcls.org/erout/goto.cfm?f=elibrary&d=28
Articles, photos, maps, and transcripts from magazines, U.S. and foreign newspapers, radio and television news programs, and reference books.
Museum of History and Industry - http://www.seattlehistory.org
The Museum of History and Industry (MOHAI) has compiled a searchable database of historical photographs. You'll find images of Seattle, King County, and other areas of Washington State. Click on Exhibits for historical information about the Northwest, and Collections for the database of historical photographs.