Joel Burslem gives the rundown on PolicyMap:

PolicyMap says they’re pulling all that data from a number of sources including the U.S. Census, Claritas, the FBI, the IRS as well as information from the Home Mortgage Disclosure Act and Boxwood Means (a real estate research firm), among others. It compiles all that data and then scrubs it to ensure that it is reliable and accurate.

Read more about it at “PolicyMap Maps A Metric Crapload of Data” including how it might be useful for real estate.