GNUnet

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

Notification infrastructure for developers.

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

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.