I write little Python scripts (or use Excel) to simulate computer/video game mechanics and quickly iterate over them before passing them off to a real programmer. I also design boardgames, so often write die rollers to calculate probabilities (it's faster for me to write something that generates hundreds of thousands of results than to actually do the math, especially if I want to tweak in real time).

My bad habit is not properly archiving these little programs, so I invariably end up recreating them from scratch each time.