Mine is ULeague (https://www.uleague.ru). It's a platform for collegiate esports in Russia. We automate Dota tournaments and organize our own collegiate events.
It was and probably is a playground to learn coding and trying new technlogies: steam bots, rabbitmq, microservice architecture, svelte, typescript. A lot of fun and experience.
We even built a seperate platform for organizers to manage the tournaments. For now we use it ourselves. Pretty cool to host Dota games from the smartphone. Players just get instant invites to the in-game lobby. And after the match is finished the score, the bracket and the stats are updated automatically.
Not much time recently for new tournaments. But very cool to get positive feedback from the community.