I do, a Shopify app that helps merchants track orders across long manufacturing/fulfillment processes. I built it ages ago when I was working as manual labor at a printing company, watching everyone run around all day trying to figure out where each order was. I'd daydream about how I could improve things and make them more efficient, write my ideas down in a notebook, then go home and build it in the evening.

I think the app is something like 12 years old now, but I still work on it to this day. I'm adding a new feature to it right now, and would like to kick the Shopify dependency someday soon.