Zulip is also open source and an alternative. It shares some of the same features as Mattermost. Mattermost seems to compete more directly with Slack; Zulip is more like IRC++. (For those from the MIT or CMU world, it's basically Zephyr with a web interface and a pile of features on top.)
Full disclosure: my company has been using Zulip since before it was F/OSS.
Can you explain how Zulip is more like a better IRC and Mattermost like Slack?
I'm a reluctant and weak (opposite of power) user of IRC and Slack and though I understand their great architectural differences, as a user they feel very similar. Surely I'm missing something.
Full disclosure: my company has been using Zulip since before it was F/OSS.