For me, it was perhaps simple-scan, a very simple and efficient GUI to scan documents. I used it with my Brother printer / scanner and it works like a charm. Especially since I do not scan stuff often, so a program with more complex UI would have the effect that I forget how to use it until the next time.
For me it was blender. I absolutely loved using blender as a teen for making silly games and animations (I wasn’t good at that). Now as an adult I re-discovered it and I use it for making DnD minifigs