Best Free & Open Source Alternatives to Slack
Looking for a free or self-hosted alternative to Slack? Below you'll find the best open source options — community-driven tools that let you own your data without vendor lock-in or recurring subscription costs.
Mattermost
Mattermost is an open source platform for secure collaboration across the entire software development lifecycle..
Alternative to: 
Discord
Discord
Raven
Simple, open source team messaging platform
Alternative to: Discord
Discord
Discourse
A platform for community discussion. Free, open, simple.
Alternative to: Discord
Discord
Zulip
Open-source team chat with a unique topic-based threading model that makes it easy to follow multiple conversations.
Alternative to: 
Slack
Slack