Population, geographic, economic and geographic data from the United States Census Bureau. Can be downloaded as excel for use in GIS.

The NHGIS is a project at the University of Minnesota designed to collect GIS data for the United States at the smallest possible Geographic level. Currently the geographic boundary files include states and counties from 1790 to the present and census tracts from 1910 to the present.

Pennsylvania's official geospatial information clearinghouse. Included are Philadelphia municipal, census, and environmental shapefiles, as well as parcel maps.