One of the things I bring out from time to time is how easy it is for companies to find information about you given your email address.
For example, if you have an uncommon email address, people can Google that. (That's easy).
Another, more advanced way is to use Facebook or Myspace. You can put in an email address, and it's easy to find people.
Okay, so many people are smart enough to hide themselves. However, there is always a more advanced way to find people. One of those ways is Pipl.com. I find that people use the same username everywhere. So, for example, if your email address is firstname.lastname@example.org, you are likely to use abc123 as your username everywhere.
You can take that username and put it in to pipl.com and it searches all over the Web for that username. Let's say you have a blog on xanga.com...or an account on flickr...this site will find all sorts of accounts associated with that username.
Be it a potential employer...or a private investigator...a lot of this information can be found for free, if you know where to look.