Zulip

Zulip fixes chat overload with threading that actually works. Every message belongs to a topic within a stream (channel). You can follow specific topics, mute others, and catch up on what matters without reading everything. It's how chat should have worked from the start. Apache 2.0. Full-featured: markdown, code blocks, integrations (GitHub, Jira, PagerDuty, 100+), search, file sharing, video calls, and mobile apps. The web app is fast. The threading model means you can be away for a day and catch up in 10 minutes by scanning topic titles.

Zulip Cloud: free for small teams (up to 10K messages searchable). Standard at $6.67/user/month for full history and features. Self-hosting is free and unlimited.

The catch: threading is Zulip's superpower and its adoption barrier. People used to Slack's flat channel model find it confusing at first. Convincing a team to switch from Slack requires everyone to learn a new interaction pattern. And the ecosystem is smaller: fewer third-party integrations, fewer bots, less community tooling than Slack.