eAthena is an open-source Ragnarok Online server emulator, and the first large software project I worked on. The project is no longer actively maintained, but active forks exist.
eAthena was my introduction to non-web programming. I joined the development team when I was 13, out of frustration with the official Ragnarok Online servers. I primarily developed NPCs and occasionally core server features under the aliases of Evera and Aria. The project served more than a hundred thousand users while I was an active developer.
My contributions can still be seen today, and it serves as a reminder for the reason why I started to pursue Computer Science. rAthena and Hercules are current active forks.