American Factfinder - Population and housing data from the 2000 Census, as well as links to the 1997 Economic Census data

United States Historical Census Browser - State and county level data from the Decennial censuses from 1790-1960

American FactFinder Reference Maps

Quick census data:

Census 2000 data

1990 Census data

1980-1990 trend changes for cities

Population Reference Bureau - Reports on U.S. and international population trends and related issues such as mortality and diseases