We tried Mattermost at work, and it was pretty terrible. Things like &quot;show new messages&quot; didn&#x27;t work, which sort of defeats the purpose of a chat app.  After that we tried RocketChat, which seems a lot closer to Slack in terms of features and appearance. From our perspective they were just too rough around the edges for functionality, with a lot of things almost-working-but-not-quite.  The caveat to all of this is that we wanted something that JustWorks, so while we did put in some effort to fix configuration and even some bugs in the software, we didn&#x27;t commit to them 100% so YMMV.<p>We started up on Zulip when it came out a few weeks back, and it&#x27;s been much better and more consistent.  Streams and topics take a while to get used to, but they make it easier to have public conversations with some semblance of organization.  If you don&#x27;t like &#x2F; can&#x27;t afford Slack, or if you have privacy needs that make it a non-starter, I would recommend giving Zulip a try.