Greener Pastures monitors the carbon efficiency of homes for sale across the US. Every day, the web app pulls data on tens of thousands of houses from the real eatate site Zillow, and calculates each home's carbon impact using Brighter Planet's cloud-based residential energy use model. The average footprint of each major city's available housing stock is graphed on the main dashboard, where users can see how rankings have changed over time.
Greener Pastures is powered by data from the EPA's US Greenhouse Gas Inventory and the EPA's eGRID database, as well as data from the EIA and other sources. As a web app, it requires no installation -- simply head to http://greenerpastur.es and start poking around.