The US Census is the leading source of quality data about the nation's people and economy. They collect data through the Decennial Census of Population and Housing, Economic Census, Census of Governments, American Community Survey and other programs.
Census Microdata Overview describes the various public use microdata (PUMS) products that are companions to the population and housing summary files produced either from Census 2000 or from the American Community Survey (ACS). This page gives information meant to be helpful in making the decision about which product to use.
Additional assistance (and data access) is provided through the Census Research Data Center for individuals with longer term research projects (as access does necessitate proposals and approvals).