A website where anyone could apply for a Google+ invite. 15k+ invites sent out in 2 weeks.
Small (5 files, 40KB), free, open source tool for creating websites. Built with PHP and jQuery. Started in 2008, meant to be a really tiny and simple alternative for creating websites. It doesn't require any setup (unzip and upload 5 files).
Created Wizards League at the age of 11, later became the biggest pool gaming league with over 10k members.