Here's a cool heatmapper: GeoIQ is an online tool for creating interactive heat maps that mashup Google Maps with numeric data and display the results in real time.
From the introduction at FortiusOne:
We see the potential of maps to become the tools of choice for people to filter, interpret and manage all kinds of location information, especially as the sheer volume and variety of geographic data explodes, now that it's generated by remote sensors, streaming video, RFID readers and GPS tracking of just about everything.
We think there's a real need to give people fast and simple but clever tools they can use to access and take advantage of all this information, and use it to solve problems and make decisions everyday, at work and at home.
The free demo lets you look make comparisons of all kinds between New York and San Francisco: income, age distribution, housing prices. I'm still figuring out how I might use this information, but it's nice to look at.