Qwitter: Find Out Who Unfollows You on Twitter
Qwitter is a very simple free tool which sends you an email when someone stops following you on Twitter. It even gives you the exact tweet which might have possibly caused you to be unfollow. Just enter your username and email to get set up. No need for passwords.
I think you should probably take this with a pinch of salt because its really very difficult to determine why someone has unfollowed you (except when you ask them directly). The exact moment after they unfollow could be based on several reasons, some of them unrelated to specific tweet.
If you get upset when people unfollow you, this will probably not be a tool that you want to use.