Zulip
Team Chat
Open-source team chat with topic-based threading, self-hosted on your own Ubuntu or Debian server.
Free self-hostedOpen source · Apache License 2.0
Also offers paid editions — see the detail.
Categories
Self-hosted alternatives to Slack
Entries in our catalogue that can take over from Slack. Each one states why, and links the vendor's own source for the claim.
Why it can replace Slack: Zulip's own why-Zulip page names Slack and argues its topic-based threading is more efficient than Slack's channel model. Source
Rocket.Chat
Team Chat
Self-hosted team chat with messaging, voice and video; free Starter and MIT Community editions, paid plans by quote.
Free self-hostedProprietaryAlso offers paid editions — see the detail.
Why it can replace Slack: The vendor documents a built-in importer that migrates users, channels, messages and shared files from a Slack workspace export. Source
Mattermost
Team Chat
Self-hosted team chat with channels, calls, playbooks and boards, as a free MIT build or a commercial subscription.
Free self-hostedOpen source · MITAlso offers paid editions — see the detail.
Why it can replace Slack: The vendor publishes a migration guide for importing a Slack workspace — users, channels, messages and files — into a self-hosted Mattermost server, using its own mmetl and mmctl tooling. Source