Data-Planet Statistical Datasets
Provides access to statistical data produced by U.S. Federal agencies, States, private organizations, and major intergovernmental organizations. There are up to 2 billion charts, maps, views, rankings, time series and tables available for use in the Data-Planet repository. Data-Planet Statistical Datasets offers powerful capabilities to compare & contrast multiple data series, perform statistical calculations on the data, and customize output views.
Many of the data sets are available for download as GIS data -- look for the "Map" icon to identify this functionality.