Maps are a mainstay of mashups. If you have geography data of any sort, chances are you might like to slap it on a map. With HeatMapAPI, you have a chance to show something a little more interesting than standard markers on your map.
The downside of this API is that calls are limited to 100 values without paying for more access. That seems like a small number, but it takes very few data points to generate a nice looking heatmap.