Feature	Zulip	Slack	Mattermost	Hipchat
Apps for every platform
Hundreds of integrations
Hosting available
Emoji reactions
Markdown formatting
Topic-based threading
Lightning-fast search
Free and open source			See below*

* On Mattermost's licensing: The components of Mattermost are, technically, released under various open source licenses. However, Mattermost's licensing and trademark policy forbids redistributing patched versions of Mattermost without renaming the project. This means, for example, that projects like Debian will never be able to include a Mattermost server package.