NHGIS (National Historical Geographic Information System)
Provides access to data and GIS files for historic census and business titles in easy-to-read charts of data and boundary file availability. Users can select the date, geography, and variable(s) for desired data. Boundary files for GIS use are available either via the Shape Finder option or in the last step of the data downloading process. Boundary files are not available for all variables and geographies. Download options include SPSS, SAS, Stata, CSV file, GIS CSV file, and shapefiles. Must register before accessing data, but there is no charge.
This site provides selected tables at the national, state, and county levels. "Map-it" feature provides links to interactive maps of the data. Data cannot be downloaded.
The Atlas contains, in both maps and text format, complete data about the creation and all subsequent changes (dated to the day) in the size, shape, and location of every county in the fifty United States and the District of Columbia. It also includes non-county areas, unsuccessful authorizations for new counties, changes in county names and organization, and the temporary attachments of non-county areas and unorganized counties to fully functioning counties. The principal sources for these data are the most authoritative available: the session laws of the colonies, territories, and states that created and changed the counties.
Pre-2010 Census Cartographic Boundary files for the Census - both political and socioeconomic boundaries available for download.