U. S. Census Bureau (data.census.gov) This is the main resource for census data from the U. S. Census Bureau, and contains census data and spatial data from the Decennial census and the American Community Survey, form about 2000 to present. The census contains general health and healthcare accessibility data especially in the ACS.
National Historical GIS [NHGIS] (via IPUMS at the Univ. of Minnesota) One of the best sources of Census data. Contains Decennial Census and American Community Survey data, along with special census like the Religious bodies census, state and local census data and more from 1790 to present.
Bureau of Transportation Statistics [via the Bureau of Transportation Statistics] Spatial data relating to rail-lines, stations, grades, bridges and more.
Inter-university Consortium for Political and Social Research (ICPSR)
ICPSR is an international consortium of more than 750 academic institutions and research organizations and maintains a data archive of more than 250,000 files relating to research in the social and behavioral sciences. The archive contains 21 specialized collections of data in education, aging, criminal justice, substance abuse, terrorism, and other fields. This resource contains datasets generated in research
