Before I got into programming as a teenager, GIMP was one of the first "advanced" pieces of software I really dove into. I learned so much just messing around, creating wallpapers, editing screenshots from games, making photo-manipulation "art", logos for imaginary companies (and later even a few real ones). I am so grateful such a wonderful, free, open source piece of software existed for me to play with, I think it's part of what sparked my interest in computing.