A guidelines app for the NHS.
My friend, a junior doctor, saw I had built a fairly crappy game, Piksel, and told me about a problem he had: junior doctors are expected to consult PDFs stored on an intranet accessible by a handful of terribly slow computers, so couldn't the solution be an app with the data stored on it instead?
I built a portal for administration in Angular 2 (later upgraded to Angular 8) and used pure CSS, too. This was built on a Kinvey backend, which stored the PDF documents as Base64 and allowed for categorisation and tagging. I built a companion app in Xamarin, complete with notifications, downloads, favourites and search.
The app was subsequently released on the App Store and Play Store and introduced to two hospitals in the UK.