Most everyone is familiar with
Google Analytics, comprehensive web tracking statistics.
CrazyEgg offers web tracking statistics in a more visual manner. When I say visual, I mean “visual.” Check out their site to see what I mean. Their software allows you to see a visual overlay of your site with a click amount next to any clickable object. They offer three different views - overlay, list and heatmap. This is a great tool for tracking which sections outperform others and therefore the info you need to help improve the less fortunate sections.
Further testing can show what different versions of a page work better than others, which ad placements give better results, which content leads to improved sales, etc. Although their service is pretty nifty, their fee is, in my opinion, a little on the high side - considering as how Google Analytics is available for anyone to use, absolutely free.