Twibbon.com - Start Something! Twitrratr. TwitLonger - When you talk too much for Twitter. Twitter Unfollower Tracker. Free to Join For users with up to 25k friends/followers, Who Unfollowed Me provides a limited functionality 'Lite' version.
There is a paid 'Pro' version that has more features and can accommodate users with up to 75k friends/followers.