HousingMaps is a Google Maps application, which allows you to search for apartments, prices, and relevant proximity of key city elements, such as transportation sources, etc.
HousingMaps uses Craigslist, a classified ads resource site, and Google Maps to provide "an aerial view" of entire cities, which aids in finding apartments, and more.
Original source: Wired News.
If you want to see another cool real estate related google maps hack check out http://www.cytadia.us where you can map your search results on a google map.