Within the strict firewall at work, we use a private Jabber/XMPP chat (most users interface with it via Pidgin) server for lots of discussions. That plus an internal install of a Discourse forum, and the effective result is pretty similar to Slack.
Within the strict firewall at work, we use a private Jabber/XMPP chat (most users interface with it via Pidgin) server for lots of discussions. That plus an internal install of a Discourse forum, and the effective result is pretty similar to Slack.