Instead of Slack

Team chat with channels, threads and an app directory.

Worth the work

A small team can leave Slack and not miss it much. A company with fifty integrations will feel the move for a quarter.

You lose: The app directory and the shared channels other companies invite you into.

Start here

Mattermost

AGPL-3.0 (open core)an eveningactive

Calls and screen share are basic; you will pair it with Jitsi for anything serious.

github.com/mattermost/mattermost mattermost.com

A compose file, a domain and a reverse proxy.

Also worth a look

ZulipApache-2.0an eveningactive
Threading is better than Slack's and nobody expects it; training is the cost.
github.com/zulip/zulip zulip.com
ElementAGPL-3.0a weekendactive
Federation is a feature and a burden. You are running a homeserver, not a chat app.
github.com/element-hq/element-web element.io
Rocket.ChatMIT (open core)a weekendactive
Heavier than it looks. The free tier of the ecosystem is thinner than the marketing.
github.com/RocketChat/Rocket.Chat rocket.chat

Facts checked 8 November 2026. Licences, activity and archive status come from each project's own repository; the verdict and the cost lines are the desk's.