If this doesn't fit, you should check out Mattermost, Rocket Chat and Zulip, all great alternatives to Slack.

Not running this but we are running a Matrix Homeserver at our place. Works great: https://matrix.org/

Then riot.im as client for the phones and web.

Not a good fit for a corporate environment, unless you're already using IRC and need a web UI.

It's basically a personal IRC client and lacks most of the features you would expect from Slack (media upload, user management, editing/deleting messages, public channels...).

Take a look at Rocket Chat and Zulip instead.

Version 3.0 releasing soon (just need to write changelog, really) has file uploading support. Not sure what “public channels” are, but you can set channels to join and lock your Lounge instance to a network.

Editing/deleting messages is a misfeature, and naturally IRC supports public channels, given that it invented them.

I mean the UI part, of course - Slack has public channels that everyone can easily discover and join.

Of course, IRC can do all of that (much better than Slack, from a technical point of view). But IRC + The Lounge is not a drop-in replacement for Slack.

You just mean recommending channels, then? Put them in your motd and a clickable list will show up when they log in.

Sure - I know more than one company who uses IRC internally, and it works just fine. However, Slack or Rocket Chat have much better usability for non-technical users.

The Lounge has easy access to IRC's LIST command, which does exactly this. The UI is a bit dirty, but we'll hopefully tidy it up soon.

https://files.yamanickill.com/Q0MSPAU8.png