Created Wizards League at the age of 11, later became the biggest pool gaming league with over 10k members.
One of the smallest (5 files, 20KB), free, open source tools 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).
Website for sending out Google+ invites to early adopters. 15k+ invites sent out in 2 weeks. Amazing community support with donations.