What Do Visitors See? Google Browser Size
Google Browser Size is a visualization tool by Google Labs that shows any website behind an overlay of browser-window-sizes average
The overlay information is actually created from the data of people who visit Google (which won't be wrong to call it an average).
The tool is perfect for learning what percentage of the visitors are seeing the most important parts at initial load & what are they missing.
When you move the mouse, a rectangle which follows the mouse pointer enables us to interact with the website. And, optionally, the opacity level can be changed.
But, like mentioned, it is created with the data from google.com & will work for websites that target average internet users. It would be great to see this integrated into Google Analytics creating a custom overlay.