The Google developers Arthur Blume and Bruno Bowden have published a little tool showcasing web designers which website parts are visible in other users’ browser windows.
To make this possible, Browser Size overlays any web site with a graph image showing browser window sizes aggregated from users. For example, you can easily recognize how many users see a download button without scrolling when it is not placed in the upper left corner. Maybe this gives you an indication why click rates at certain positions are underestimatedly low.
Browser Size always refers to the visible website without any window toolbars and sidebars.
The tool is available at http://browsersize.googlelabs.com/.