[cwbuilds]

Is there an 'open source Slack' type thing anywhere?

This is the closest I've found- https://mattermost.com/

[srid]

If you are willing to explore a hybrid model of chat, there is Zulip: https://github.com/zulip/zulip

[elkos]

I'm really happy with Matrix chat. But it's more of a protocol than a sole implementation. The good people at Element are pretty neat too.

[ensignavenger]

Zulip is another one, mentioned in a sibling comment. Matrix/Element is used by some companies. Nextcloud Chat exists, but I don't know much about it.

[Ruthalas]

I think the Nextcloud offering is called "Nextcloud Talk", for anyone wanting to check that out.

[ok_dad]

IRC probably. It doesn’t have every feature slack has though.