Pointers to other interesting files and sites are also often included. For each federal census between 1790 and 1900, this article by LaVona L. Ness tells what information was collected. Comments are made about the limitations of the census. In four parts, a list of all the counties in the United States, plus the names of their county seats and the zip code of the county seat.
States are in alphabetical order by their two letter postal abbreviation.
From Alaska through Illinois plus Adams County in Indiana. From Allen County in Indiana through Prentiss County, Mississippi. A list of all the counties in the United States, in a single file. It's set up as a shell script for Unix, so you can run it and have a file built for each state. Genealogical and historical societies of the United States with a nation-wide scope.