I recently discovered Neighboroo.com while searching for a google maps application to find crime data. What an incredibly easy site to use. Lots of data here too that you can browse visually. You can find pretty much identical data at city-data.com and even get specific by zipcode. City-data isn't quite as fun though.

That being said.. my other two favorite google map apps are zillow and mapmyrun. Both incredibly useful. I wish google maps had a simplified mapmyrun type thing on it by default... it would be very handy.