This app finds users that do not follow you back and allows you to unfollow them at once for free.
There is also an auto follow feature that follows accounts that interact with search terms you specify, and unfollows them if they dont follow you back in 5 days.
Note: If you have reached your twitter follow limit, and the functions of this app are not working, manually unfollow accounts until you are back under your follow limit, then try using this app again.