The library has a detailed research guide, Finding Census Data that provides links to online data as well as listing our print holdings for both the population and economic censuses.
Social Explorer is a user friendly database for both downloading and mapping US Census data. Social Explorer contains census data from 1790 to present (as the database announced that 2010 Census is now available as of March 25th).
If you are looking to compare United States demographics with other countries, try Passport GMID (Global Market Information Database) produced by Euromonitor. We especially recommend the Consumer Lifestyles reports, which give detailed information on consumer demographics in over 70 countries.