MattermostCoordinate work through channels, messages and shared files.
Source published on GitHub under a custom/non-standard licence (GitHub shows no standard SPDX id); check terms for commercial use.
Team chat
Free alternatives to Slack
Find a free app for the parts of Slack you actually use.
Compare the free allowance and limitations on each app’s page. These are workflow alternatives, rather than identical copies.
Free options
Suggest an alternative →Mattermost
ZulipOrganise team conversations into streams and focused topic threads.
Free and open source (Apache-2.0 licence per GitHub).