Zulip

Zulip is a team chat application built around topic-based threading: every message belongs to a channel and a topic within it, so multiple ongoing conversations stay organized and easy to catch up on instead of collapsing into a single interleaved stream. That structure is aimed at making both live, synchronous discussion and slower, asynchronous follow-up work well in the same tool, rather than treating chat as purely real-time.

It's self-hostable (Docker or a native installer) or available as a hosted service, includes a full REST API and bots/integrations framework, and supports end-to-end encrypted search across message history. The project is built by a distributed open source community — over 1,500 contributors and 99+ people with 100+ merged commits each, per the project's own numbers.

Zulip fits teams and open source projects that have outgrown a single flat chat channel and want conversations to stay navigable as the team or contributor base grows, without giving up real-time chat.