What it is
Kanto is a forum where people make their own communities, each with its own rules, tags, layout and mod team. Threads and replies can be posted anonymously where the community allows it, and there's voting, mentions, notifications, DMs, follows and a personalised feed on top of that.
How it went
I built it in about five weeks, close to 500 commits. A lot of that time went into the stuff nobody sees: moderation and reporting, rate limiting, privacy and data deletion, encrypted DMs, permissions, and a full admin panel. There's also a thing called micro-sites, where you can host a small single-file HTML/CSS/JS project in a sandbox right on the forum.
It's not online at the moment, but it taught me a ton about building something where other people's content and safety are part of the job.