The National Historical Geographic Information System (NHGIS) provides population, housing, agricultural, and economic data, along with GIS-compatible boundary files, for geographic units in the United States from 1790 to the present.
Series of representative samples drawn from the U.S. censuses of 1850 to 2000 and the Puerto Rican censuses of 1910 to 2000. Includes American Community Survey (ACS) data from 2000 to 2006. BEST source for accessing the American Community Survey microdata!
Partnership between the U.S. Census Bureau and Duke University in cooperation with University of North Carolina System. Census Bureau provides access to unpublished microdata from the economic and demographic censuses and surveys via a secure computer laboratory located at Duke University. Confidential data from other government agencies may be accessed as well; including data from the National Center for Health Statistics (NCHS) and the Agency for Healthcare Research and Quality (AHRQ).