I built something that changed my friend group's social fabric (via) I absolutely love this as an illustration of the thing where the tiniest design decisions in software can have an outsized effect on the world.
Dan Petrolito noticed that his friend group weren't chatting to each other using voice chat on their Discord server because they usually weren't online at the same time. He wired up a ~20 lines of Python Discord bot to turn people joining the voice channel into a message that could be received as a notification and had a huge uptick in conversations between the group, lasting several years.