Find out where your visitors click
There are several commercial services that add a so called heatmap to your website. This map is build by analyzing all clicks that visitors make on your website. If you analyze the hotspots over a certain period of time you will probably be able to optimize ad and content placement on your website.
ClickHeat is the first heatmap that is free of charge, at least the first that I heard of. Your server needs to support PHP (5) and the graphic library GD to be able to run the script. Once it is installed you can take a look at certain pages, group pages, a set data, a data range and how different browsers performed.