Compared to most of these comments, I've not built anything impactful. But the software I've written that seems to have gotten most use is SDL-Ball and the Fin

Compared to most of these comments, I've not built anything impactful. But the software I've written that seems to have gotten most use is SDL-Ball and the FinalKey password manager. Well, I also built a "digital bulletin board" for a youth org back when PHP was in fashion, it's no longer used, but they used it, and bought minor upgrades for almost 15 years, so I like to think it had a positive impact on that org. They ended up primarily using a booking system that we designed together exactly to fit their needs.