NHGIS is a one stop location for many of your historical (1790-2000) U.S. Census and boundary file needs. This site is well organized, making the data collection and download process quite simple. There are numerous pages giving descriptions, source material, and useful external links for all the data that the site has for free download. In order to obtain the data or shapefiles that you desire, the NHGIS site has two main search engines, which require you to registered as a NHGIS user. (Simply click on the link to complete the free registration process.)
The first method of obtaining data is the NHGIS Shape Finder (NOTE: you will be asked on log in) which provides boundary files for use in GIS. When using this search process, the Shape Finder prompts you with questions, as seen above. The final output is a downloadable zip file containing the desired boundary shapefile(s).
The second method of obtaining data is the NHGIS Data Finder. The Data Finder will provide you with datasets related to certain attributes which you desire. The entire process for finding the aggregate data is slightly more complicated than the three step shapefile finder displayed above, but the results are astounding. Go step-by-step through the detailed questionnaire to acquire data related to the United States Census dating back to 1790. After selecting the year of the data desired, you can chose the geographic unit of interest (the entire United States, individual states, counties or various other attributes). From there the data can be personalized based on numerous characteristics as can be seen in the screenshot to the left.
Once all of the desired attributes have been selected and submitted, the site compiles the data and outputs the information in a comma separated format, which is perfect for export into Microsoft Excel.
For more information regarding the site, including tables of all the data that the site has to offer and assistance on how to import and join NHGIS data into GIS software, visit NHGIS -Mapping Data.