Real eye-tracking studies use special headsets to track where the user looks. However, you can do this virtually - by tracking the users mouse clicks. There's open source software to that will build a heat map for you, based on where users click.
I've run this on some of my pages - it only works on pages that are not completely fluid layouts (unlike GotAds, eg. where the page resizes to the whole width of the brower), but it is cool.
Two advantages to doing this on your own server side: it's faster, and you don't send your valuable referer data to anyone else.