Census Bureau's main interface for retrieving demographic data from the American Community Survey, Decennial Census, and their Population Estimates. Can get complicated to use -- see Help to view tutorials.
Census Bureau publications on a wide variety of social and economic characteristics of the population, such as household composition, ethnicity, and income. In-depth reports are prepared on important social, economic, and demographic issues.
Census Bureau data, repackaged by the Social Science Data Analysis Network at the University of Michigan for the purpose of spotting trends. Offers charts, maps, rankings, and (for American cities) measures of racial segregation
Collection of data measures spanning all U.S. states, counties, cities, metropolitan areas, and zip codes. Data goes back more than 20 years. Covers the economy, education, crime, government finance, health, population, religion, social welfare, and transportation.