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

NextCloud

Free open source personal cloud software for sharing files, collaborative editing, instant messaging, personal music streaming and lots of other things. It’s extremely flexible thanks to lots of add-on apps that can be installed through the graphical interface. Nextcloud instances can be accessed through web browsers or dedicated apps for all major platforms. You can host a cloud yourself or sign up on an existing instance.

Fediverse address:
@nextcloud@mastodon.xyz

Collabora Office

Online fork of the LibreOffice productivity suite aimed at businesses, including long term tech support, admin tools, smartphone apps and cloud collaboration.

Fediverse address:
@CollaboraOffice@mastodon.social

Its development company, Collabora, is one of the biggest upstream contributors to LibreOffice’s source code, and has a close relationship with the FOSS community.

Fediverse address:
@collabora@floss.social