I build things that can help others have sex with strangers. I can quite clearly remember the first time someone came up and told me they were getting married because of my spare-time project.
My first big side project was that; I have received over a 1000 cards/mails/etc of people who are married and/or have babies because of that side project I created with my wife on vacation on the canary islands. We expected it wouldn't get more than a handful of members, but it had 10.000 after a few months and kept growing. Very satisfying feeling for something just built for fun.