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 is a joint project between the Bureau of Labor Statistics and the U.S. Census Bureau. The CPS is a monthly survey that collects basic socio-demographic information, labor force characteristics, and economic status. To access CPS data files, users can download the data from an FTP site or use various tools for creating tables from the microdata. Alternately, they can go to the National Bureau of Economic Research's CPS site or to IPUMS-CPS.
IPUMS (Integrated Public Use Microdata Series) USA
The IPUMS project at the University of Minnesota is an excellent source for Census data in the form of microdata samples from each decennial Census from 1850-2000 and from the American Community Survey for 2001 and onwards. See https://usa.ipums.org/usa/sampdesc.shtml for a list of the samples available via IPUMS. The data include standardized/harmonized variables for topics such as industry, occupation, race and ethnicity, and educational attainment, for easier comparisons of data over time. The microdata come with geographic tools and boundary files, to assist with geographic analyses of the individual-level data. IPUMS-USA is one of many IPUMS efforts - see https://www.ipums.org/ for the full list.
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, economic data on businesses, crime data, health indicators, 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.