Zulip

Zulip is a powerful open-source group chat application used by thousands of teams for business, open source projects, and communities. What sets Zulip apart from Slack and other chat tools is its unique threading model: messages are organized into streams (channels) and topics, making it easy to catch up on conversations without being overwhelmed.

This topic-based threading means that even after a day away, you can scan topic names in each stream to decide what to read, rather than scrolling through endless chat history. Zulip also features keyboard-driven navigation, powerful search, message editing, file sharing, integrations with over 100 services, and a full REST API.

For self-hosting, Zulip is well-documented and runs on any Linux server. It scales from small teams to large organizations and is actively developed. Major organizations like Wikimedia, Mozilla, and Khan Academy use Zulip. It offers full export capabilities, making it easy to migrate your data.