My open data side project (which incidentally had interest from Apple and Google at the time) had a need for a couple of web server certificates for the API and website, so as yet another side project I built a little certificate management tool and forgot about it for a year.
After a year I then went back and realized it was getting 1,000 downloads a week, I did it a bit more work on it and stuck a price tag on it, it's since been my full time job for several years.
After a year I then went back and realized it was getting 1,000 downloads a week, I did it a bit more work on it and stuck a price tag on it, it's since been my full time job for several years.