"Zillow allows users to see the value of millions of homes across the United States, not just those up for sale. In addition to giving value estimates of homes, it offers several unique features including value changes of each home in a given time frame (such as 1, 5, or 10 years), aerial views of homes, and prices of homes in the area. Where it can access appropriate data, it also provides basic information on a given home, such as square footage and the number of bedrooms and bathrooms. Users can also get current estimates of homes if there was a significant change made, such as a recently remodeled kitchen. Zillow provides an application programming interface (API) and developer support network.
"As a part of its API, Zillow assigns a numerical integer to each of the 70 million homes in its database, which is plainly visible as CGI parameters to the URLs to individual entries on its website. The identifier is not obfuscated and is assigned in sequence for each house or condo on the side of a street. Zillow reports on individual units, such as providing street address, latitude and longitude. When integrated with the features of a typical online reverse telephone directory and wiki-mapping services such as WikiMapia, it allows for nationwide "seating assignments" of U.S. neighborhoods for each house that has a listed phone number with a real human name" (Wikipedia article on Zillow.com.)