Making quake mods when I was 12 got me interested in programming. I think my last one was 'Tellytubbies: Must Die'. It was fun making maps, models, textures and editing sounds of teletubbies in pain.
Mapping and modding Quake 2 and 3 pushed me from Basic and HTML/CSS into much less forgiving C/C++ programming around the same age -- wouldn't be the engineer I am without it. It's invaluable to modify something you know and understand and extremely rewarding, which is the first thing I teach to friends who want to learn.