Zillow

I think that it is OK to take publically available information such as Zillow did. Zillow takes the estimates of every house's value in the country, and the houses for sale as well.  You can view the houses with their prices on a map, so you can then compare a house for sale with other houses in the neighborhood to compare with value.  This is helpful, because when looking for houses in the newspaper, all you know is the location and the price.  Zillow gives the user much more awareness about the house and it's comparison, so a potential buyer knows if they have a good deal or not.