Almost a year ago, I posted my open source side project (an AWS security scanner): CloudSploit[1]. Since then, I have kept the open source version, but built an entire web service around it with paid plans. It's gotten a good amount of traction and I've enjoyed adding new features, as well as the process of learning to start a new business (it's all new for me).

[1] https://news.ycombinator.com/item?id=10062746