In my double life as an actor, I've written some software that greatly simplifies the main day to day task of running a talent agency. Its ~15 users love it to death, but that's it, it has a total of ~15 users. It's my happy little project.

Could it be useful to more people? Almost certainly, and at some point I considered running it as a service, and I even had a few trial users. But then I realized that dealing with GDPR compliance and the like wasn't going to be as fun, so in the end it remained an internal project.