Louisville Metro Mapper - Here is a great Google Maps implementation for the city of Louisville, Kentucky. It packs in maps for Crime, Sex Offenders, Traffic Cams, Homes for Sale and Restaurants. The use of info-windows is quite unique in this mashup.
Map Game Day - This user generated map mashup lets you create map markers to show popular campus locations, and game day maps (like ticket locations, parking, tailgate parties etc..) for college campuses.
Mp3 Audio Walking Tours: NYC - Google Maps is used to map available mp3 audio walking tours for New York City. New clips for other U.S. cities are also detailed with a map on the soundsforsights blog.
WikiBroker Real Estate Maps - Search for home values and prices of sold houses by state, city, neighborhood, street or zip using keywords. Results are displayed on a Google Map.