Trulia ia a real estate search engine with some interesting features. Their mission is to “helps you find homes for sale and provides real estate information at the local level” and their business model is based on advertising. It has a good map based interface.
“Trulia Hindsight is an animated map of homes in the United States from Trulia. The animations use the year the properties were built to show the growth of streets, neighborhoods and cities over time.”
The visualization is based on public property assessor records for properties in Trulia’s database, presumable because they have been offered for sale in the bast five years. These records typically include the date that a house was built. As an example, look at this animation for the Baltimore area.