This site contains U.S. and state boundary maps (counties, congressional districts, census tracts, etc.) that are available for exporting. A majority of the files are available in ArcInfo formats.Center for International Earth Science Information Network (CIESIN)
CIESIN is the World Data Center for Human Interactions in the Environment. This site provides FTP access to international GIS datasets. CIESIN Gateway also provides search and retrieval of SEDAC's (Socioeconomic Data and Applications Center) Data and Information Catalog Services. Some of the datasets available are: Geocorr (Geographic Correspondence Engine which allows users to create correlation lists between various U.S. geographies); Archive of Census Related Products; Gridded Population of the World; Global Population Database Public Use Microdata Samples (PUMS) (microdata from the decennial U.S. censuses of1970-1990); and Social Indicators of Development (an Interactive query of the World Bank's dataset containing 125 socioeconomic variables from more than 170 economies).
The National Geophysical Data Center offers:
Datasets in the GED are defined by authorship and title and grouped in databases according to geographical coverage, georeference system, and projection. Resolution of data will vary between global, regional, and local databases, as well as within a dataset; however, all grids and reference systems are geographically compatible within a database group. All datasets are in a common GIS structure and accompanied by appropriate documentation.Guide to Mostly Online and Mostly Free U.S. Geospatial and Attribute Data
This guide includes links to sites at the state, U.S., and international level. Also includes links for a subject-based approach.National Spatial Data Clearinghouse
Users can search for data in the Clearinghouse and browse related information in registered Clearinghouse web sites.Soil Survey Geographic (SSURGO) Database
This database is provided by the Natural Resources Conservation Service (NRCS) of the US Department of Agriculture. Data can be downloaded in either DLG-3 Optional format or ArcInfo export format.EarthExplorer
EarthExplorer was developed by the U.S. Geological Survey (USGS) for use by scientists seeking sources of information about the Earth's land surfaces. EarthExplorer contains metadata (descriptive information about datasets) and offers online samples of earth science data that may be ordered through the system.USGS Mapping Science Software
A collection of public-domain software developed by or for the USGS. This site contains cartographic, geospatial metadata, and mapping product applications.USGS Geographic Data Download
GeoData offers U.S. Geological Survey data files for use in geographical information systems for analysis and integration with other geospatial data. The data files are not directly viewable using a WWW browser or image viewing tools. Data files are offered from the following series:
The BTS Office of Geographic Information Services is a national resource for transportation spatial data and GIS in transportation (GIS-T) information. The Geospatial Data section includes the National Transportation Atlas. The atlas is a collection of geospatial databases developed by the Department of Transportation and other federal agencies depicting transportation facilities, networks, and services of national significance.