People who use historical census data may be interested in the following web site:

The United States Historical Census Data Browser
http://fisher.lib.virginia.edu/census/

It is sponsored by the Inter-University Consortium for Political and Social Research. Data is available at the state and county level for the years 1790 through 1960, and of course, the data provided varies from census to census. But it is a useful place to look if you need to know, say, how many slaves there were in Ulster County in 1790 (2,914), how many women were employed in manufacturing in the New York State in 1860 (53,227, compared to 176,885 men), or how many Italian-born residents there were in the New York State in 1920 (545,173, of which 184,546 were in Manhattan, compared to 344 in the state of South Carolina).