Looking for GIS data for the U.S.? The Berkeley/Penn Urban and Environmental Modeler's Datakit has just been released. The site contains more than 150 downloadable ArcMap-ready shapefiles and raster datasets for the 48 contiguous United States.
The data were produced at the Institute of Urban and Regional Development (IURD) at the University of California, Berkeley and the Penn Institute of Urban Research (Penn IUR) at the University of Pennsylvania, in cooperation with Penn's Cartographic Modeling Lab (CML).
The data is free, and all interested urban and environmental planners, analysts, modelers and enthusiasts are encouraged to utilize the site to further narrow traditional disciplinary gaps between urban and environmental planning researchers/practitioners. It claims to be the first site to bring together spatially comprehensive and comparable urban and environmental GIS data.
Users who uncover problems (other than the fact that "Modeler" is mis-spelled on the title banner) or might wish to add their own national data to the website should e-mail John Landis at firstname.lastname@example.org.