Links and resources for working with US Census data. Also, check out Harvard University's Guide to U.S. Census Data.
County Business Patterns: Industry level employment data at the US county level.
American Community Survey: Ongoing survey of community demographics and economic information at the community level
Fact Finder : A Searchable Database of Data sets from U.S. Census Bureau
US Agriculture Census: Collection of Ag. Census Resources
Social Explorer: This resource provides maps detailing demographic data in the US at various geographic levels.