Zulip Alternatives for Linux

Skype is an instant messaging and VOIP (Voice Over Internet Protocol) client to communicate with people in a distant place.

Telegram is a free cloud-based instant messaging platform.

Slack is a cloud-based team collaboration tool.

Tox is a peer-to-peer instant-messaging and video-calling protocol that offers end-to-end encryption.

Trillian is a proprietary multiprotocol instant messaging application.

Viber Free Messenger and Phone Calls.

Discord is a freeware proprietary VoIP application designed for gaming communities.

Signal is the most scalable encryption tool.

Empathy is an instant messaging (IM) and voice over IP (VoIP) client which supports text, voice, video, file transfers, and inter-application communication over various IM protocols.

Mattermost is an open source, private cloud Slack-alternative, Workplace messaging for web, PCs and phones. MIT-licensed.

Wire is a cross-platform, encrypted instant messaging client created by Wire Swiss. It is available for iOS, Android, Linux, Windows, macOS and Web browser clients.

ICQ is an open source instant messaging software.

Rocket.Chat free open source chat solution.

Instantbird is a cross-platform instant messaging client based on Mozillas XULRunner and the open source library libpurple used in Pidgin.

Matrix is an open protocol for real-time communication. It is designed to allow users with accounts at one communications service provider to communicate with users of a different service provider via online chat, Voice over IP, and Videotelephony.

Zoiper is an easy to use sip video softphone, with excellent voice quality and easy to setup.

QuteCom (WengoPhone) is a free-software SIP-compliant VoIP client.

Dialog Messenger is a handy and feature-rich enterprise multi-device messenger available for server or cloud.

Wickr is an instant messenger application.

Gitter is an open source instant messaging and chat room system for developers and users of GitHub repositories.

Tkabber is a GPL instant messaging client for the XMPP protocol which uses the Tk toolkit for the GUI.

Toky is an automation platform.