I develop Cursive - https://cursive-ide.com, a plugin for Clojure development in IntelliJ. I started working on it seriously in 2014, started selling it in 2015 after about 2 years in beta (during which time I had a daughter) and it has provided all my income since then. The sales are more than my salary + bonus (but less than total comp) at my last job at Google. This year is the first year that sales have dipped slightly, probably due to COVID and a better competitor for VS Code, but it's still very profitable.