Umbrella Consulting was recently contracted by the good folks at WalkScore to work with them as they created a detailed walkability ranking system for the top 40 cities in the nation. 

This project entailed analyzing of millions of data points around the country, aggregating and analyzing summary statistics in PostGIS, and using software like Mapserver and Tilecache to create heatmap visualizations.  They've now integrated all this into their existing Google Maps infrastructure, and you can view the results at walkscore.com. 

If you're coming to the FOSS4G conference this year, we'll be presenting a paper on the methodology.