My wife always compained that there was no suitable time tracking app for breastfeeding our little baby. Within three weeks I developed a simple app with proposals from her which fits her needs.
After a few weeks using it we were suddenly overwhelmed with inquiries from our friends searching for such an app as well.. Within another week, I made it ready for the AppStore and the PlayStore and now it does around 400 € a month :-)
https://www.stillapp.de - although the page is in german the app is translated into english as well.