Remember the feature of the old iPods where you could add notes that used a subset of HTML? When I was in high school I wrote a little command-line program that would generate a bunch of notes linking to each other that in effect let you play blackjack on your iPod. It was a total hack, but I had a casino contact me asking to sponsor it. It didn't end up happening, but it's the closest I've come to accidentally making money.

I've written other open-source software since and had people offer money for new features, but I'm not sure that counts since it involves additional work.