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. For data prior to 2013 see the US Census Bureau Statistical Abstracts of the United States.
American FactFinder http://factfinder2.census.gov
Start here for population, housing, economic data.
People and Household http://www.census.gov/people/
Census Reports http://www.census.gov/prod/www/
CenStats Database http://censtats.census.gov/
Data sets, tract searching, demographics and economic/business data.
Map Stats http://www.fedstats.gov/qf/
Click on your location on the map and get quick county level demographic snapshots and links to more detailed tables.
U.S. Foreign Born Population http://www.census.gov/population/foreign/
Historical Census Data http://www.census.gov/population/www/censusdata/hiscendata.html
Historical census reports in PDF for 1790-1860, 1990, and 2000 and Measuring America which contains questionnaires and instructions, plus individual histories of each census.
Historical Statistics of the U.S. http://www.census.gov/prod/www/statistical_abstract.html
Contains statistical abstracts from Colonial Times to 1970.
Historical Census Browser http://fisher.lib.virginia.edu/collections/stats/histcensus/
Allows users to view state and county data for a wide range of Censuses, track topics over time, or generate thematic maps. This is not the Census of names and it cannot be used to track individuals.