The New York Times has published an amazingly detailed interactive 2010 U.S. census map. The application lets you explore information about population growth and densities, ethnic concentrations and housing development patterns in a series of choropleth maps.
Depending on the zoom level you can view statistics at the county or census tract level. With each area shaded according to the value of the selected variable. Hovering the mouse over a region displays a pop-up with exact numbers and other details.