Zulip
Apache-2.0 team chat organised by topic threads; self-host or use Zulip Cloud
Comms / Realtime · Apache-2.0 · 26.1k
Reviewed 2026-10-21 · last push 2026-10-21 · repo checked 2026-10-21
What's Good
Apache-2.0 (zulip/zulip), server release 12.3 (2026-09-21), commit on 2026-10-21. ★~26k (~45 a week), developed since 2015. Useful for remote or open-source teams that want a Slack alternative they can self-host, with a REST API, bots and webhook integrations.
The Catch
The self-hosted server is a substantial install (Postgres, RabbitMQ, Redis, memcached, nginx) and wants its own VM. Mobile push notifications from a self-hosted server go through Zulip's push service, which is only free up to a certain size. The topic model takes a week or two to get used to.
Verdict
The best open-source Slack alternative if your team likes threads.
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