Often its quite hard to figure out that how many other website is ran by a particular a website owner or blogger, specially if the said person wishes to stay anonymous.
eWhoIS uses an interesting method to cross reference websites using Google Analytics or Adsense publisher ID to match and figure out the similar websites.
Most of the website owners of bloggers uses single Google adsense or analytics account to monitor or monetize their website, and eWhoIS takes advantage of this for Reverse Adsense ID Lookup and Reverse Google Analytics ID Lookup.
In the above example, I have done an eWhoIS on a popular Windows news and discussion website WinVistaClub.com to find out what other website is ran by the owner of WinVistaClub.com. The tool detected and reported a list of 5 other website using the same adsense / analytics account.