Ask HN: What interesting software did you work on in the past?

This comment https://news.ycombinator.com/item?id=12037548 by lisper about his time writing a patch for the software running on an instrument on the Galileo spacecraft makes me wonder how many other interesting stories HN members have.

So, What interesting software did you work on in the past?

I don't care if it is famous or if it made a billion $, just interesting.