One of my favorite sites is the US Census homepage.
The site won’t win any awards for being user friendly, but if you spend some time with it, you can get detailed information about the US population.
The American Factfinder area is the place to go for these statistics. You can quickly pull up fact sheets on states and communities, but the real power comes from looking at the Data Sets. Choose the Decennial Census from the menu of the left hand side of the page and then pick one of the Summary Files (SF 1 is a good place to start). Click on Detailed Tables and then use the menus to look up data.
For more information, visit our reference desk. -AKaiser