This tool estimates the amount of click traffic (and revenue, optionally) your site could receive from ranking at positions 1-10 in Google in one of two ways:
Click distribution results (traffic volume and revenue, optionally) are estimated using a distribution model taken from a set of leaked AOL data in 2006. We also know AOL's search is (and was at the time) powered by Google. You can find the leaked data mirrored here, it has ~20 million web queries collected from 650K web users over three months. You can also read all about the leak scandal... and other scary points in the data.
We've got a video posted on how to use the tool, go check it out.