If you're talking about a mobile app, then maybe Twitter Check is what you're looking for.
It lets you "Track new and lost friends and followers on Twitter (to track Twitter, Instagram, Facebook and LinkedIn, use our other app, Friend Check)" and it is available on Apple's app store: https://itunes.apple.com/us/app/twitter-check-manage-your/id587686670?mt=8
Otherwise, if you're talking about browser-based, the only thing that comes to mind is Twitual, Twitter Karma, and FriendOrFollow...
They're all applications that pretty much do the same thing: Enter a username, find out whether they're following you (and if not, unfollow them!!)
