S Next-Gen self-hosted chat server and clients.
R Communications platform that puts data protection first (alternative to Gitter.im and Slack).
Notification infrastructure for developers.
M Platform for secure collaboration across the entire software development lifecycle, can be integrated with Gitlab (alternative to Slack).
C Customer communication platform (alternative to Intercom and Zendesk).
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.