Zillow’s got ’em. So does Trulia. Crazy Egg will even map your web site activity with them. Basically, any data set can be made visually sexier with a heatmap.
I suspect we’ll be seeing more and more heat map implementations come to market very soon; housing prices, rental prices, cost of living, census data, weather trends, financial data – all can be displayed via a heat map. Consolidating this complex information into a visual representation highlights subtle patterns and illustrates trends very effectively.
Plus it just looks cool (or hot).