Data.census.gov is a platform to access data and digital content from the U.S. Census Bureau. It combines several previous census tools like American Fact Finder, Census Business Builder, DataFerret and Current Population Survey. Contains data beginning in 2010.

Years: 1790-2000
Geographies: all (down to blocks)
Formats: varies; often can download to SAS, SPSS, Stata

Years: Samples from fifteen federal censuses (1850-2000) and the American Community Surveys (2000-2006).
Geographies: Public use microdata areas
Formats: Can be downloaded with syntax files for SAS, SPSS, Stata
Notes: These are microdata, as distinct from the aggregate (published) data found in nearly every other census data resource. See What are microdata? for details.

Years: 2000, post-2000 estimates and projections
Geographies: states, cities, counties, tracts, block groups, ZIP codes
Subscription resource with data on demographics, health, education, inequality, and more. For additional information about using PolicyMap: http://guides.library.yale.edu/policymap.
Formats: export maps as GIFs or PDFs; export shapefiles; export data to Excel.

Years: 2000, post-2000 estimates and projections
Geographies: states, cities, counties, tracts, block groups, ZIP codes
Formats: export maps as GIFs or PDFs; export shapefiles; export data to Excel.
Notes: includes business and marketing data.

Years: 1790-2000 (decennial census), 2005-2012 (ACS)
Geographies: all, down to block groups
Formats: Download data to Excel, SAS, SPSS, Stata; download maps as images

The ProQuest Statistical Abstract of the United States is the authoritative and comprehensive summary of statistics on the social, political, and economic conditions of the United States, and will continue where the U.S. Census Bureau publication ceased.

Years: 1790-2000
Geographies: primarily national; some largest metropolitan areas and largest cities; urban-rural; census regions
Formats: PDF

Years: 1790-2000
Geographies: varies; recent reports down to places
Formats: PDF
Notes: These are digitized versions of print decennial census volumes. Volumes include narrative introductions for each census about changed methodologies, underlying laws, etc.

Years: 1970-2000; 2006/2011 estimates and projections
Geographies: all (down to blocks). Some products provide data normalized across years.
Formats: Download data to DBF or CSV; download shapefiles or MID/MIF MapInfo
Notes: Not all products are available online, and online products work in Internet Explorer only

Years: 1790-2000; coverage varies by topic.
Geographies: primarily national, some states, metropolitan areas, cities, urban-rural
Formats: tables, essays, can download to PDF or Excel
Notes: Covers a wide range of topics; statistics are drawn from many sources (not just the decennial census).

Years: 1790-2000
Geographies: all (down to blocks)
Formats: GIS-compatible boundary files. Data can be downloaded for import into Excel, SAS, SPSS, or Stata.

Years: 1790-2000
Geographies: nation, regions, states, counties, metropolitan areas, largest cities, coastal regions, places
Formats: PDF, HTML
Notes: Easy to scan by time period and geography, but only a subset of the "Decennial Census of Population and Housing Reports (Census Bureau)"

The Statistical Abstract of the United States, published since 1878, is the authoritative and comprehensive summary of statistics on the social, political, and economic organization of the United States.