(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.