World and US Population Clocks from the Census Bureau
American FactFinder -- Census Bureau's start screen for searching census data
Population Profile of the US, latest edition - features only yearly data (advanced users may go to American Fact Finder if they are looking for some kind of time series)
Social Explorer - Census tracts for every ten years since 1940 with population variables expressed as a percentage. Choose year and then variable. Zooming into an area reveals more detail, include tract number and street name. {National Science Foundation and New York Times)
Current Population Survey - many are available in Tutt Library Docs C 3.186:.
Government Information Sharing Project - USA Counties, Census of Population and Housing, Population Estimates by Age, Sex and Race, Equal Employment Opportunity File, School District Data Book Profiles, Regional Economic Information System, Economic Census, Census of Agriculture, U.S. Imports/Exports History, Consolidated Federal Funds Report, Earnings by Occupation and Education.
Historical Statistics of the US, Colonial Times to 1970 - selected reports covering historical Decennial Censuses (Population and Housing Counts)
Census Data - for the U.S. 1790-1960
Histpop -Online Historical Population Reports Website -- not a government site
Population Clock (Population Coalition)