The Best Viral Idea I May Have Ever Seen
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.