Qwitter: Catching Twitter quitters

Qwitter e-mails you when someone stops following you on Twitter with a message like this:

John Gruber (gruber) stopped following you on Twitter after you posted this tweet:
What's the difference between Arial and Helvetica?
Just tell us the Twitter username you want to track and your e-mail address and we’ll do the rest.

Qwitter: Catching Twitter quitters