OpenStreetMap Calendar
osmcal.org
osmcal.org
A couple of years ago we had a hand-fiddled table in our OpenStreetMap wiki with events, but discovery was poor and nobody really wanted to write wiki table syntax _shudder_ to add events. So people either didn't do it or only advertised their events in their group chats or Facebook groups.
Thus I started writing some code, and now people can add an event by just logging in with their OpenStreetMap account and filling out a small form. We still display our events in a Wiki by using the API and a small Mediawiki plugin to maintain human compatibility for people who are used to getting events from the wiki. There are also RSS feeds, iCalendar subscriptions and event file downloads, so that everybody can subscribe to events in their preferred form.
It's a small project that I've been running for a couple years and it's really rewarding to see people using it every day.
* https://github.com/Everything-Open-Source/open-source-events * https://foss.events