I saw no mention of these Slack alternatives (all Open-Source):<p>* Matrix (<a href="https://matrix.org" rel="nofollow">https://matrix.org</a>)<p>* Mastodon (<a href="https://joinmastodon.org/" rel="nofollow">https://joinmastodon.org/</a>)<p>Both could be put to use as a Slack alternative, but lack a number of the bells and whistles that make Slack an easy experience.<p>Mastodon in particular I could see replacing Slack, if it became more polished and they made an organization-centric version/mode of their UI.