For those who haven't tried it, here's a video of how Eclipse Foundation uses Mattermost as an open source, self-hosted Slack-alternative: https://www.youtube.com/watch?v=dfeNFJnCfcg

While there's not yet C64 support, Mattermost connects to IRC, Slack, Gitter, Discord, Telegram and HipChat using Matterbridge (https://github.com/42wim/matterbridge).

In terms of features, the Mattermost community has added quite a few that aren't available in Slack--markdown support, multi-team accounts, threaded messaging, etc.: https://www.mattermost.org/what-slack-might-learn-from-its-o...

Mattermost deploys as a single Linux binary with MySQL or Postgres and you control everything.

One issue we had with Mattermost is that some authentication modules (e.g. OAuth) are not part of the free core. Rocket Chat does not have this issue, though the mobile app is a bit less polished.