I see quite a few stories like this on HN. They are v intriguing. Incredibly high return on time-investment, allowing you the freedom to work on other things.

I used to think my cash cow (freelance management consulting and putting together pitch deck presentations) was pretty awesome. But examples like this dwarf it in terms of ROI ratio.

Can you give more detail on what it is this business actually does? Or else, do you have other examples of high ROI side projects?