Birthday Alarm. You go to the website, it crawls through your address book, sends out an email to all your friends and simply asks them for their birthday. Free sign-up, or users pay $14/yr to have ecards they can send to those friends.
Such a simple idea. Has 50mm users right now grossing $4mm revenues/year. Had 50mm more users years ago — might not be an opportunity to get those other 50mm (given Facebook, Bebo, MySpace these days), but I think there is the opportunity still.
More info at TechCrunch.