American Fact Finder
The American FactFinder is an extensive source for census statistics from the U.S. Census Bureau. Users can create data tables from the Decennial Census (2000 and 2010), the American Community Survey (2000-present), and the Economic Census (1997, 2002, 2007, and 2012) and download those tables into spreadsheet files. See http://factfinder2.census.gov/faces/nav/jsf/pages/what_we_provide.xhtml for a list of available data collections.
Current Population Survey (CPS)
The Current Population Survey, a joint project between the Bureau of Labor Statistics and the U.S. Census Bureau, is a monthly survey that collects basic socio-demographic information and labor force characteristics along with various supplemental studies on matters such as voter registration, internet usage, school enrollment, and fertility. To access CPS data files, users can go through the Data Ferrett. Alternately, they can go to the National Bureau of Economic Research's CPS site here or to the IPUMS CPS site here.
Integrated Public Use Microdata Series (IPUMS)
The IPUMS project at the University of Minnesota is an excellent source for historical U.S. Census data, in the form of collections such as microdata samples from the Decennial Census for 1850 onwards and tabulated Census data for various levels of geography for 1790 onwards. The database also hosts a growing collection of microdata census files from foreign countries that can be accessed at https://international.ipums.org/international/.
Social Explorer provides quick and easy access to current and historical census data and demographic information. Its contents include the entire U.S. Census from 1790 to 2010, annual updates from the American Community Survey, data on religious congregations for the United States for 2009, decennial religious congregation data for 1980-2010, and carbon emissions data for 2002. Users can create reports and maps at various levels of geography, including counties, Census tracts, Census block groups, and zip codes, depending on data availability. Social Explorer is also available via Databases at Emory.