United States Census Bureau – Maps & Data – access to geographic data and products such as the TIGER/Line Shapefiles, KMLs, TIGERweb, cartographic boundary files, geographic relationship files, and reference and thematic maps – GIS Tutorial: Downloading and Prepping Census Data
National Historical Geographic Information System (NHGIS) provides aggregate Census data and GIS-compatible boundary files for the United States between 1790 and 2012.
data.gov – Data.gov provides the ability to search approved data from hundreds of sources including federal government, cities, counties, states, and universities.
TIGER/Line Files allows users to download various versions of the Census Bureau’s TIGER (Topologically Integrated Geographic Encoding and Referencing) database
Social Explorer (United States Interactive Demographic Information Maps) is dedicated to providing easy access to demographic information about the US through interactive data maps. User can use Social Explorer to make maps and reports, visualize/analyze US demography, and explore historical demographic data.