Created the first version of an app that took large long PDFs of train driver/conductor schedules in my country and presented them in an nice way in an app. The previous sources were all hard to navigate and had ads. I spent a weekend parsing PDFs into SQLite, and shipped the app with a preloaded database. Then kept updating and tweaking and added some fun intersections of the data ("who else is operating this train").

8 years later my source of data stopped providing data because of data protection concerns and the company introduced their own app/functionality for the same. Never earned me a dime directly but did get me my next job - it was a great sample for the interview