ClickHeat is a free PHP script that creates a visual heatmap of clicks on a given web page. It uses AJAX to quickly log clicks, and then it has a very nice admin console to display various heatmaps. This is a free alternative to those paid heatmap services like CrazyEgg, and it delivers fairly well. You may need to tweak the javascript to make the variable clickHeatServer point to a full url if you want to have the script hosted on one site and available to be used from other domains, like I have done. Otherwise it looks for a "clickheat" folder on whatever domain the script is being called from.

This is something I had planned on making, and it's good to see a nice solution is already available. If would be nice if it used MySQL to store data, instead of a log folder and a cache folder. But you can't argue with success, and it won't be overloading a MySQL database anytime soon.

ClickHeat: free clicks heatmap