A lot of demographic data collected and used comes from the US Census Bureau. While the major census takes place every 10 years, the American Community Survey, American Housing Survey, Population Estimates, and other smaller scale surveys provide a wealth of data. Use the Quick Facts search tool for easy access.
RAND State Statistics is an online source of 200 data series for all states, and over 150 additional data series for California. RAND selects statistics that most impact public policy, and can be browsed in the following main categories: Business & Economics, Health & Health Care, Population & Demographics, Crimes, Prisons, & Courts, Federal Government, State & Local Government, Government Finance, K-12 Education, Higher Education, Income, Wealth & Poverty, Energy, Environment, Transportation, Labor Force.