Google Adsense Tracker is a powerful php script that keeps detailed logs of all impressions and clicks on AdSense ads on all your websites without altering the ad code itself. The data can then be used to analyze the effectiveness of your sites, track different ad sizes and styles, or even individual pages.
You can track every click-through so you'll know what your visitors are looking for. This makes it easy for you to build more perfectly targeted, profitable pages.
It can track unlimited domains and pages. It's resource intensive and should be hosted separately.
If you just have a small site you probably don't need it. AdSense Tracker is a tool for professionals.