This is an old post from 2009. As such, it might not be relevant anymore.
This is a pretty handy tool. It places an overlay above your layout so you can see roughly what percentage of people will be able to view parts of your site. Now, there are some caveats here, the first being that if you have a wide browser and testing a margin:auto layout then you need to resize your browser to see realistic results results. Not to mention all this should be taken with a pinch of salt as there is a Myth of the fold in modern Web design.