NHGIS (National Historical Geographic Information System)
"The National Historical Geographic Information System (NHGIS) is a project created to freely disseminate a database incorporating all available aggregate census information for the United States between 1790 and 2010."
The NHGIS consists of three major components:
The great bulk of the United States summary census data exist in machine-readable form, but they were largely inaccessible. Over 670 gigabytes of data covering the period 1790 through 2010 exist or are in preparation, but they are scattered across dozens of archives and stored in incompatible formats on CD-ROM, magnetic tape, or paper.
Only a small fraction of these data are available on the Internet, and even those offer only primitive documentation an extraction tools. Moreover, census summary data cannot be effectively exploited without clear definitions of each geographic unit, but high-quality electronic boundary files exist only for the 1990 census year."
You MUST establish an account to download any data.
To see what data is available, click here.
Don't forget to cite the Center when using their data.
The NHGIS is made possible by the Minnesota Population Center at the University of Minnesota, Twin Cities campus.