Slack Alternatives for Mac OS X

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

Jitsi (formerly SIP Communicator) is a free and open source multiplatform voice (VoIP), videoconferencing and instant messaging application.

Glip is fully searchable, real-time group chat; video chat, task management, file sharing app.

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.

Rocket.Chat free open source chat solution.

eXo Platform is an open source, standard-based, Enterprise Social Collaboration Platform.

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.

TribeScale is an All-in-One platform to coordinate your team around what really matters - communication, goals, decisions, tasks and results.

Priority Matrix is a time management software application that is supported on a number of platforms, including Microsoft Windows, Mac OS X, Android, and iOS. It is based on the Eisenhower Method of arranging tasks by urgency and importance in a 2x2 matrix.

Flock is a communication app for teams. Packed with tons of productivity features, Flock drives efficiency and boosts speed of execution.

Grape is an REST-like API framework for Ruby. It's designed to run on Rack or complement existing web application frameworks such as Rails and Sinatra by providing a simple DSL to easily develop RESTful APIs.

Fleep enables communication within and across organizations, be it your team chats, project communication or 1:1 conversations.

Zulip is a chat and collaborative software.

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.

Hubgets provides chat, voice and video turning team communication.