Feature	Zulip	Slack	Mattermost	Hipchat
Apps for every platform
Hundreds of integrations
Convenient SaaS offering
Gmail-style threading
Markdown formatting
Lightning-fast search
Emoji reactions
Free and open source			See below*
Actually increases productivity :)

* On Mattermost's licensing: The components of Mattermost are, technically, released under open source licenses. However, Mattermost was founded by an ex-Microsoft team, is developed primarily by paid employees, and their licensing and trademark policies do not allow third parties to distribute patched Mattermost servers without renaming the project in their fork. In contrast, Zulip is a true open source project with a wide development community and a clean Apache 2.0 license.