zulip
topic-threaded team chat
public
registration: invite only
threaded chat
Zulip is open-source team chat organized by topic. Instead of a single scrolling firehose, every conversation lives under a named topic inside a channel - so you can catch up on exactly the threads you care about and skip the rest. It's the async-friendly answer to Slack and Discord.
details
- Registration: invite only. You need an invitation to the organization - there's no open sign-up.
- Model: topic-based threading keeps long-running discussions readable and easy to pick back up.
- Open source: self-hosted, no seat limits, no data harvesting.