I like www.housingmaps.com Its cool, simple and it just works.
The idea of Mashing up data from Craigslist and Google Maps its just great.
I came across a great set of geospatial mashups in government agencies at http://www.fargeo.com/casestudies/category/enterprise-mashup. The list includes 'City of San Francisco Bridge Preventative Maintenance Program Web Application', 'City of San Mateo Master Address Database', 'City of Boston Redevelopment Authority: Web Based GIS Redevelopment Project Tracking Portal' and a few others.
Here's some good government-focused mashups: http://blog.programmableweb.com/2008/10/23/use-of-mashups-in-the-us-government/. Mashups in development for federal agencies include Housing and Urban Development (HUD), the California Health Care Foundatation (CHCF), and the Defense Intelligence Agency (DIA).



I am still partial to Zillow: www.zillow.com.