Seconded. We can't use public services for internal messaging, but we do have JIRA + Stash inside the firewall. Hipchat server would be good, or is it XMPP compatible?
It's definitely XMPP compatible. We have an outside dev firm that does some work for us and they connected their own chat system via XMPP. It works fine for them as far as I can tell. Their help docs [1] [2] have all the details you need.
