Twitter - follow me follow you?

I'm often curious as to where I get some of my followers and I'd like to compare my followers with some other folk to see if I'm right in assuming I get some of my followers via them.

Enter this cool app called Followerlap. All you have to do is type two twitter ids, it doesn't have to be yours but it can be. So, if I want to see how many of my followers are also followers of, for example, Digital Photography School, I add:

HelenBradley and DigitalPS and click Submit.

Really it couldn't be easier. In a few seconds I know that nearly 20% of my followers also follow DigitalPS. Pretty neat hey?