This isn’t anywhere near as impressive as some of the examples here, but anyway:

I run an Etsy store that sells artwork. One of the most time-consuming tasks was creating the mock-ups for product listings, so I wrote a shell script that used Imagemagick to automatically generate all the images.

I was pretty pleased as I’d never written a Bash script before and it saves hours and hours of work.

I also wrote a Ruby script that uses the Etsy API to generate a fully-working Jekyll e-commerce site from my Etsy store’s listings. Much better that paying Shopify plus a bunch of other apps for pretty much the same thing.