American Factfinder  (U.S. Census Bureau)
http://factfinder.census.gov

City-Data.com  (Extensive regional demographic data.  Operated by Advameg, Inc.)
http://www.city-data.com

USA Today
http://www.usatoday.com/news/census/index.htm

Association of Bay Area Governments (ABAG) Data Center
http://www.abag.ca.gov/abag/overview/datacenter/databod.html

Counting California
http://countingcalifornia.cdlib.org/title/castat.html

United States Historical Census Browser
http://fisher.lib.virginia.edu/census/

California Department of Finance
http://www.dof.ca.gov/HTML/FS_DATA/Fs_home.htm

Bureau of Economic Analysis  (US Department of Commerce)
http://www.bea.gov/

National Center for Education Statistics  (School District Data)
http://www.nces.ed.gov/fastfacts/

Federal Interagency Forum on Aging-Related Statistics
http://www.agingstats.gov/agingstatsdotnet/main_site/default.aspx