Los Angeles Times Homicide Map
The Los Angeles Times Homicide Map is the map interface of the LA Times' Homicide Report. It is the bast mash-up of Google Maps and data that I have seen. Users can filter the data in a number of ways; by race, gender, age, cause of death and day of week and see the results instantly on a Google Map. This throws up some interesting facts, for example Sunday is by far the most popular day for murderers (or is it their least popular day).
Being a map based interface you can also filter the data by geographical area and find results within a user defined distance of a user defined location. Furthermore you can then take the results in the form of a GeoRSS feed or KML to view in Google Earth.
Here is a map of homicides in Compton in the first six months of this year (and speaking as a European this looks frightening).