Jan.11/06: Corrected this post (Removed references to Yahoo! Real Estate)
Here is a new Google Maps Real Estate mashup to add to the mix: HomePriceMaps.com. HomePriceMaps is a "freely browsable database of homesale prices" in places like New York, Philadelphia, Hawaii, Miami and more. It is a mashup of home sale price data integrated with Google Maps.
Take a look:
(**I have noticed that with Firefox 1.0.7 this mashup bogs my browser down a bit. I might suggest viewing and using this mashup using a recent version of Microsoft Internet Explorer instead of Firefox)
Google Maps Mania has an index of many more Google Maps Housing & Real Estate mashups found here.