Rocket.Chat 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

Full team-collaboration platform you can host yourself. Its license (MIT) is permissive: free to use, modify and redistribute — including inside commercial and proprietary products.

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.

An open-source alternative to

Other alternatives worth a look