When you go to Alexa and search for a domain, it now also displays all the sites that are owned by that person/company. It doesn't always work, but sometimes it does show a whole network of sites.
Nice way to check out what network the competition owns, but the downside is that they can do the same thing for you. I personally don't mind, but i'm sure some people would prefer to keep their network private.
Alexa - Amazon.com
Alexa - Problogger.net
Bookmarks