(UIUC Subscription) 1790-2010+. Data from the decennial Census and the American Community Survey, with a user-friendly interface. Allows the creation of maps & statistical tables. Census Tract boundaries are available back to 1940. Also includes religion surveys from 1980-2010, which are not affiliated with the U.S. Census. (Note: this service is provided through the university, but you must make an account. This database has a limit of 5 simultaneous users.)
2000-2019. Data from the decennial Census and the American Community Survey about households, education, income, homeownership, business, and more. Important note: as of July 2019, no new data will be added to FactFinder. FactFinder is being replaced by data.census.gov
The Integrated Public Use Microdata Series (IPUMS-USA) consists of more than fifty high-precision samples of the American population drawn from 15 federal censuses and from the American Community Surveys of 2000-2010.
ICPSR archives and disseminates census data acquired from the United States Census Bureau as well as files prepared by ICPSR and other principal investigators. A good place to look for decennial census data.
For archived data files from older census' that are no longer available through American FactFinder or data.census.gov, check out this list of downloadable files. Be aware that the files do not have a lot of accompanied information so you may have to dig around to find what you need.
Maps bring data to life! The Census Bureau's Geography Program website provides resources for getting started with mapping your census data including: mapping files, reference maps, and reference files.
The US Census Bureau does not always use commonly understood vocabulary to define their variables. It is important to know what the census terms mean. Use this handy glossary to look up the official definition for words and terms used by the Census Bureau.