[himujjal]

What are the good open source alternatives out there?

DuckDuckGo'd a few ones. But not convincing ones.

[BiteCode_dev]

I was surprised how much I liked zulip. The threading model and the policy of few notifications by default speaks to me.

The UI is not the most modern but the software is productive and friendly for a chat application, particularly when you look for an old information burried in the history.

[adioe3]

I've used https://rocket.chat/ in one company and it was perfectly fine.

[ikbenkenobi]

Self-hosted Mattermost?

[encom]

IRC.