Top Alternatives to Mattermost

Slack is a cloud-based team collaboration tool.

Viber Free Messenger and Phone Calls.

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.

fluxday is a user-friendly, free & opensource task & productivity management tool for growing startups.

Rocket.Chat free open source chat solution.

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.

Let's Chat is a persistent messaging application that runs on Node.js and MongoDB. It's designed to be easily deployable and fits well with small, intimate teams.

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

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.

Zulip is a chat and collaborative software.

Flowdock is a group chat for teams which integrates with GitHub, Jira, Trello.

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

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

Hack Chat is a minimal, distraction-free chat application.

YakYak is a desktop chat client for Google Hangouts.