Gancio

A free open source collaborative online calendar. It allows local communities to share a common calendar that anyone can post events to, even people without an account. (Posts are moderated to prevent spam though.) You can also interact with Gancio through the Fediverse, and it has RSS support for people who want to follow through feed readers.

Fediverse address:
@gancio@mastodon.cisti.org

KDE

Worldwide free open source software community which creates all kinds of popular useful projects such as the Plasma desktop, Kdenlive, Krita and lots more.

Fediverse address (main):
@kde@floss.social

Fediverse address (videos):
@kde@tube.kockatoo.org

Fediverse address (conference videos):
@kde@conf.tube

Fediverse address (Spanish language community):
@kde_espana@floss.social

Fediverse address (French language community):
@kdefr@framapiaf.org