FoldSpy is a tool for finding out the actual dimensions of your visitor’s browser window. It gives you access to this data and presents it in completely new way, the data from FoldSpy is overlayed on your site. FoldSpy is most useful for people who need real data to make important layout decisions. It can answer questions like “where do I put this ad if I want 85% of my visitors to see it without scrolling?” Now, you can find the answer in seconds and because the FoldSpy data is displayed in-place, on your site, the answer can be viewed in a way that’s most useful.

Requirements: Modern Browsers with Javascript Enabled
Demo: http://www.foldspy.com/overview/
License: License Free