The National Historical Geographic Information System has recently announced its initial release of time series tables of U.S. Census data.
Freely available to the public through the NHGIS Data Finder, these datasets provide aggregated tables of statistics comparable across time, and for many scales of geographic area. For instance, the initial data release includes 100% population count data from every Decennial Census since 1790 at the state and county levels, while providing such data down to the Census tract and place levels for the 1970 through 2010 Decennial Censuses.
Time series data can be viewed and downloaded in layouts differentiated either by file (one file for each time point of data, encompassing all geographic units) or by row (one file for all geographic units, with each time point occupying a row of data). Future data releases will not only provide column-differentiated time series data, but will also incorporate the American Community Survey datasets.
The National Historical Geographic Information System is a free-of-charge and public-accessible service of the Minnesota Population Center, a demographic research center at the University of Minnesota.