I just learned of an excellent website, Social Explorer, that generates zoomable demographic maps of many types based upon 2000 U.S. Census data as well as earlier data. It's quite telling to look at the distribution of the black population throughout the United States, for example, or to zoom in and get a close look at how much segregation still exists in your favorite city. Or see where Swedish immigrants are concentrated. Or which counties have the highest concentration of health care workers. There are also a few slide shows available, like the black population of New York City from 1910 to 2000. There is so much to explore that I could really lose a whole afternoon there if I'm not careful.
Below, the density of people holding PhD degrees in and around Boston. The darkest color on this map represents 15-25% of the population.