Unfollowers.me lets you see who has unfollowed you on twitter. We use optional email or tweet notifications to notify you if someone has unfollowed you.
"Welcome to fllwrs, the Twitter follower tracker. Would you like to see a record of who follows and unfollows you every day?
Would you like to get notified when someone unfollows you?
Use fllwrs to monitor changes in your Twitter followers over time and keep a history of followers that have been lost or added."