HipChat
We've got full Atlassian here, on-prem because that's where it's at these days. Would probably switch to Slack if they offered an on-prem version.
HipChat is ok but the UI isn't anywhere near customisable enough for my liking. I wish it was a bit better to use with any XMPP client but so much of its functionality is based around the web app core.
Biting the hand that feeds IT
