Mattermost is a free, open-source alternative to Slack, Microsoft Teams. It can be used without subscription fees — and its source code is open for anyone to inspect, modify and self-host.

About

Self-hosted team messaging built for security-conscious orgs. Its license (MIT / AGPL) is a copyleft license: you can use, study and modify it freely, and derivative works must remain open-source under the same terms. Commercial use is allowed.

What switching gets you

• No subscription, no per-seat pricing — free forever, for individuals and companies alike.
• Your data stays with you: it can run locally or on your own servers, with no vendor lock-in.
• The source code is public — auditable, extensible and maintained in the open by its community.

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