Qwitter e-mails you when someone stops following you on Twitter with a message like this:
John Gruber (gruber) stopped following you on Twitter after you posted this tweet:
What’s the difference between Arial and Helvetica?
Nice idea for a useful service (wish there could be the same thing for blog subscriptions), a bit of humor from the designers, and somehow they manage to do it without even asking you for your password. Cool.