This widget works based on the number of comments on your posts. It takes the most commented post and gives it a value of 1. You can choose the thresholds for the three tiers. It applies css classes to the tiers in the following manner: fire, hot, medium, and mild. You can use the classes to color the post titles, background etc to show "heat map." Depending on how many posts, you may not see all of the classes.
I use it on http://matthewaprice.com in the sidebar. I used to color the li bullets with the css classes, but i no longer use them. They are still applied though.