My last job hosted an XMPP server, and we also had a locally hosted pastebin. On the XMPP server we hosted chatrooms, so the IT people would hang out in one, and the software devs in another.
What I use right now is Microsoft Lync (aka Communicator), since we're on the MS stack for infrastructure. The nice thing about that is you can click the "Share Desktop" button if you needed to show something you have. I don't know if it supports predefined chatrooms, but you can get multi-user chats by dragging people into IM conversations.