Zulip

Zulip is an open-source team collaboration and chat application that combines the real-time presence of chat with the organized threading model of email. Every channel in Zulip is subdivided into distinct topics, allowing team members to follow, catch up on, and search relevant discussions without getting overwhelmed by a continuous stream of unthreaded messages.

Because it prevents conversation fragmentation and context loss, Zulip is especially popular among open-source software communities, scientific research collectives, and distributed civil society organisations operating across multiple time zones. It can be self-hosted on standard Linux servers.