I built stratascratch.com working a full-time job and teaching at a university 1-2x a week in the evenings. The idea came from the amount of headaches and inefficiencies I experienced trying to teach non-technical students how to code. I built a platform that made my life easier. It's fairly profitable but not at a scale where I can quit my day job. Since I don't have much free time, I have to hire several freelancers to help with product improvements and sales.

For the record, I don't charge my students to use the platform :P