I'm quite young so this happened very recently. Me and my friend created an Android app for students that could let you view our school's online workbook (so grades, lessons, news etc) in the app without opening and logging in the website. Some time passed and the workbook gained popularity among other schools (mostly because it was open source) and so did our app. Then a school asked us to change the ui a bit and paid us 80€. I think I used my 40€ in pc parts and other stuff :) Soon after we stopped working on it and graduated.
There are still like 200~ users using the app even after 2 years since we stopped updating (and it crashes a lot).