I used heat maps extensively to map the intensity of the location of an area with WhatHood and used them again with the Septa regional rail crime heat map. My method was to use heatmap.js and Leaflet. As far as I knew, this was the only open source option.
I stumbled onto the Google Maps heat map API while trying to figure out how WalkScore was displaying heat maps. It looks like it’s been around for at least a year. Rochcass has a good starter for getting up to speed with it. After I give it a try, I’ll post a comparison to heatmap.js. One thing heatmap.js is lacking is that it’s not actively being developed.