This is a commercial site which charges a fee for obtaining directory information. I have only listed it here because it is useful for identifying repositories that have historical city directories. Some of the repositories have digitized collections that are freely available on the web.
A rich collection of research materials for genealogists and historians to trace family histories and American culture. Includes full-image population schedules from the U.S. Census (1790-1950), U.S. City Directories (1821-1989), U.S. Freedman's Bank Records (1861-1875), U.S. Indian Census Rolls (1885-1940), Canadian Census (1825-1921) and more.