This tool will give you idea about how many % of user are not able to see which part of your website.
This is develope by googler on his 20% time. If you want to see which part of page in your web site is used more/less this would do the job.
Read more about this tool from its developer:
http://googlecode.blogspot.com/2009/12/introducing-google-browser-size.html
Link to the tool is here: http://browsersize.googlelabs.com/
