Tox

R

Rocket.Chat

rocket.chat

Communications platform that puts data protection first (alternative to Gitter.im and Slack).

Notification infrastructure for developers.

M

Mattermost

mattermost.com

Platform for secure collaboration across the entire software development lifecycle, can be integrated with Gitlab (alternative to Slack).

C

Chatwoot

chatwoot.com

Customer communication platform (alternative to Intercom and Zendesk).

N

ntfy

ntfy.sh

Push notifications to phone or desktop using HTTP PUT/POST, with Android app, CLI and web app, similar to Pushover and Gotify.

Zulip is a powerful, open source group chat application.