We use Pidgin Openfire Instant Messaging Administration in the organization that I work for.
Ramsey- We use <a href=”http://messenger.softros.com/”>SoftrosLANmessenger
One cool thing is that you can also share files of any size across the intranet. It is quick and easy to setup. Licenses are like $10 a pop and the Server Admin Plateform is not too expensive.