A childhood friend has a car garage. He wanted to give a paper to his customers to show the work that had been done, what they asked for etc. There were no free AND easy to use solutions for one man show type of business. So i built something that fit his needs, with his input. It's a simple form with 6 steps that you can print at any stage. I put it on a domain under his name so he knows where to find it at all times. For some reason google picked it up and it's now the first search result. It has over a thousand returning users(at least yearly) and a few hundred MAU. It's still free, people send me emails/call me to tell me how useful it is and how much they love it, some ask "is it really free" - it's under firebase so no costs to me either, other than occasional bugs and "features".

Another thing i built which had a bigger impact was a covid "get outside motivation paper" during lockdowns. I had the "form technology" from the previous project and i just adapted it for the covid needs and added a signature for touch displays. You could generate a pdf with all legally required fields and sign it and print/show it to the authorities, on your phone. It had half a million users over a few months, then the government set up their own. I even had a GDPR authority contacting me thinking i'm stealing people's data etc. I actually showed them that the PDF is generated locally, there was no database and once the site loaded, you could even generate it offline. They did not follow up.

Another thing i built, with questionable impact, was a election fraud checker. The government had a API and feeds data every 5 minutes with number of people who voted at all locations. I wanted to see this in real time with a GUI/graph to see who registers more votes per minute than the actual systems allow(tablets). I actually found some locations that had bursts of hundreds of votes in a few minutes, that was physically impossible. Sad to say i did not do anything with the data(although it was public at that time).

Some of the most impactful things i built.

Edit: typos