Pidgin is a desktop chat program that lets you log into accounts on multiple chat networks simultaneously. This means that you can chat with friends on XMPP and sit in an IRC channel at the same time.
Zulip is a powerful, open-source group chat application that combines the immediacy of real-time chat with the productivity benefits of threaded conversations.
Twake is an open-source suite of collaborative tools, team messaging, group channels, task management, a calendar, a drive to share and store your files, real-time document collaboration, and end-to-end encrypted video meetings.
Chanty is an all-in-one team collaboration tool that helps teams collaborate and communicate via real-time instant messaging, voice, and video calls. Administrators have full control over the channel and the members who use it.
Slack is a messaging app for businesses that connect teams to the information they need. Slack allowing to work together in different channels that can be organized as you like.
Flock is a team communication app and online collaboration platform with shared notes, reminders, to-dos, polls, video calls, and more.