4 out of 5 stars (based on 1 review)
Mattermost is an open-source, self-hostable online chat service with file sharing, search, and integrations. It is designed as an internal chat for organisations and companies, and mostly markets itself as an open-source alternative to Slack and Microsoft Teams.
On the website there are different models/packages on sale: https://mattermost.com/pricing/
However the software is open-source and can be hosted on your own server, for example on a Debian system: https://docs.mattermost.com/install/install-debian.html?src=dl – therefore this software can be considered as trade-free.