Who.Unfollowed.Me is a Twitter Application to track unfollowers.
When you Sign in with Twitter, and check 'Who Unfollowed Me' I ask twitter for a list of your followers. If it's your first time using Who.Unfollowed.Me, I simply store that list of followers until the next time you come back, otherwise I take the new list of followers, and the old list of followers and compare the two. Pretty simple.
I've always believed that Twitter should be a two way street, and not a numbers game. If you follow me, and I follow back, then you decide to end the conversation (unfollow me) than I should reciprocate and unfollow back. Twitter never made this an easy task, and it got harder as the followers/following numbers increased. Who.Unfollowed.Me was just an easy way for me to find the unfollowers quickly and efficiently.