Overlaying custom information or crowd sourced information where a location based decision has to be made is very interesting. I recently did an entry on eRealtor market in India and I feel the real estate websites can make use of such maps in different ways. The possibilities are many and can help provide differentiation.
Where else such a map can be used? What other location based searches that you do? Can this be used for hotel bookings and travel planning websites where customers can share their experiences? Imagine a map where you can clearly identify which is a good location to stay and which is not. Now zoom out at state or country level and you know which is a good place to make travel plans etc.