I built an accounting app :

https://compta.libremen.com

Did not intend to at first, but having quit Windows, I had to replace the Access application I had built with an accountant friend of mine to manage my small business. Looking around the web, I could not find something I liked.

I ended up re-writing the thing as an SaaS for my own use, but then figured that if I'm using it, others might want to, and all I had to do was to add an 'id_client' field to the database. It turns out to be a bit more complicated than that, as users tend to come with different needs, and to think of ways of using your app you did not anticipate.

Also, when I tell an accountant that I wrote an accounting application, his eyes pop, his head tilts slightly, and he appears to be evaluating whether I'm the dangerous kind of crazy before ending the conversation. Must be due to the horrors they have to deal with all day.

It's a hard sell, revenue is closer to $500/year than $500/month, because I have very low fees and very small clients. But I take great pride in counting one accountant using it in his professional practice. It's also great fun to tune as the number of lines grows, however odd this may sound.