Mattermost desktop applications are available for Windows, Mac and Linux operating systems. They support all the features of the web experience, plus:
Connect to multiple Mattermost servers from a single interface, and switch with shortcut keys.
Auto-start Mattermost when a user logs into their machine
(Linux) Desktop Entry for the application to more easily integrate into a desktop environment
Homepage : https://about.mattermost.com/
Open source : Yes