Unfollowers.me lets you see who has unfollowed you on twitter. We use optional email or tweet notifications to notify you if someone has unfollowed you.
Until the Twitter team can get the service working again for good, here's what they should strongly consider: Close the site. Take it offline. Put plywood over the doors and windows, as it were, with a big "We're remodeling!" sign on the front. Ask users if they want to be e-mailed when the site reopens for business and don't send that e-mail until the thing is fixed. Really fixed. Then have a grand reopening party.