This chrome extension keeps track of the accounts on twitter as you follow and unfollow them. If you have the nagging suspicion that an account has been unfollowed, but not by you, you'll be able to verify.
This extension does not save any data to the cloud. The list of accounts you follow is saved in your browser itself. No data in your twitter account is ever altered.
Available in the chrome store: twitter friends
I used the code from chrome-extension-template as a starting point.