Ask HN: Your most successful side project?

I know I -- and I assume many others -- would be interested to hear about HNers' most successful side projects.

Wherein side project is defined as something started by you -- with or without others' help -- on the side of a full-time dayjob that turns/turned a decent profit, was sold for a decent amount or has considerable uptake and a bright future.

I'll go first. In 2005 I started a designer community event called CSS Reboot that, in its 1 1/2 years under my ownership, made $5,000 in ad sales. Fall 2006 I then sold it for $10,000.