I built a dozen or so educational apps. I started each on iOS then have ported a few to Android and Windows Phone. It hasn't yet been a great financial success, but it is almost entirely passive income now. Updating apps for UI changes like iOS 7, the iPhone 5 screen size, Holo, etc. are kind of a pain but not bad at all compared to constantly selling myself on new consulting gigs or chasing down non-paying customers.