I came across this page from an Open University blog claiming Google Analytics can be used to view third-party sites.

http://blogs.open.ac.uk/Maths/ajh59/008701.html

I never tried this method yet but would be interested to see how you guys think about it.

What he does is gather data from his own visit to that site, or anyone using his greasemonkey script, it is by no means the visitor data for the complete site...