Have you ever wondered why one day you have 100 followers, but the next you're down to 95? Twitter doesn't notify you when someone stops following you, and it can be cumbersome to comb through your followers list to figure out who dropped you.
If you're somewhat of a masochist and want to know when and why someone stops following you, Qwitter is the tool for you. Just sign up on the Qwitter Web site, and Qwitter will monitor your account. If someone "unfollows" you after you post something, you should get an e-mail with the username of the person who dropped you, as well as the post that may have inspired them to leave.
Qwitter can be a useful tool if you're someone who is concerned about inadvertently offending people with your tweets. Knowing what particular tweet may have prompted someone to drop you can be helpful. You might even send a friendly message to the person that unfollowed you to iron out any misunderstandings. Of course, conversely, if you unfollow someone, you should be aware that they may be using Qwitter as well, and they may have a few questions for you.
Perhaps sometimes, ignorance really is bliss.