My most impactful project was definitely NoCoin [0], the first web miner blocker. Back when Monero miners started appearing and sneakily mining on pages, like The Pirate Bay for example, I decided to throw together a browser extension that would simply block requests to the resources that hosted these mining scripts. The project was in no way a technical achievement, it was simply intercepting requests and blocking them based on a list. I could have very well added the resources to some other project like uBlock origin. But it got traction, ended up in the press (WIRED, Motherboard, Gizmodo) and ultimately started being integrated within browsers (Opera was first) and most of the popular ad blockers. The project lost its relevance as everyone else was doing it better and maintaining the list of blocked resources was too time consuming for me. Nevertheless, the goal was achieved, which was to "get rid" of crypto mining on the web. The mission got carried by bigger actors, which brings me more satisfaction than the popularity of the project.
Another impactful project of mine was also a browser extension. Internal tool that started as a lunch time project to make my team's life easier. Can't go in detail on that one, but basically they liked it and start suggesting improvements. So did a department that was working with us. And bit by bit, it became a really useful tool that became standard in these departments. Last I heard, the tool is now deployed company-wide. Crazy to think it started just from some lunch time hacking :-)
[0] https://github.com/keraf/NoCoin