Crazy Egg is a wicked service that allows you to measure where people click on your site. I have no idea how they do it. You basically install a piece of Javascript in your HTML code which must somehow hijack the mouseclicks or something and report this back to a Crazyegg database.
What really makes this tool shine is the Heatmap. The Overlay and List views are in comparison pretty boring and for that you might as well use standard web statistics tool for that. I'm not going to explain what the Heatmap is. Just look at the attached screenshot from a report I've made based on www.issuetrackerproduct.com