ColoradoHomeStop.com is a cool real estate mashup with great Google Maps work:
ColoradoHomeStop.com features a great mapping implementation which pulls in data from WhereToLive.com and displays it using Google Maps. The user interface is amazing and it employs some interesting Google Maps development to display hundreds of map markers. The creator of this mashup explains: "It uses the default Google Maps API with some custom AJAX work to plot markers server-side, dramatically increasing the number of markers that can be shown on the map. This solves the problem with Google Maps getting bogged down by large numbers of markers." To give it a try, run a detailed home search (if you're a serious Colorado home buyer) or directly click the "Search" button under the "Quick Search" area on the main page. Nice work!
More Housing and Real Estate Mashups can be found here and here..