My day job is all about doing presentations, so I obviously wrote my own slide software. It is a pure expression of my workflow, my requirements, and my idiosyncrasies and the way all of the above changed over ~10 years. It is a great performance booster if you can basically brain dump your thoughts straight into slides, and if you can just implement breaking changes (or choose to keep supporting an objectively bad feature for all eternity). I don't need to care about any other users, or the bus factor.

The only part of that software that ever became public was the animated syntax highlighter[0], but I don't believe any other part would help anyone else accomplish anything.

[0] https://code.movie/