In my current job I authored and maintain a tool that is used organization wide and saves several hundred man hours a year. Unfortunately since it has proprietary logic I can't host it on git, but I'd love to mention it on my resume. Thing is, I built in my spare time, a coffee break hear, waiting for a windows update there.
I'm quite proud of it, and really think that it is one of my better achievements with the company -- how would I list it on my resume?