Mattermost.

Open-source messaging platform for developer and DevOps teams

Facts

BigBlueButton (self-host)
Virtual classroom and video conferencing; self-hostable
EU · open-source
↗
CryptPad
End-to-end encrypted collaboration (FR/EU).
FR · open-source
↗
FluffyChat
Open-source Matrix chat client with focus on simplicity and privacy
DE · open-source
↗
HedgeDoc
Markdown collaboration; self-host.
DE · open-source
↗
Huly
Open-source project management and team collaboration platform
PT · open-source
↗
HumHub
German open-source social intranet and collaboration platform
DE · open-source
↗
Kopano
Dutch groupware with self-host options
NL · open-source
↗
Nextcloud
Open-source self-hosted collaboration platform for file sync, chat, and office
DE · open-source
↗