It's now easy and free to setup an internal (intranet) chat server, for your office or at any firm you required. if you have fully qualified domain name running on your server then you can setup it over the internet. for internal use, the fully qualified domain name is not required.

Openfire is free opersource application that provides user friendly administration of your chat server. it's easy to intall and manage. unlike jabber it's installation & confugration is totaly web based. supports all databases for the backend. Openfire is incredibly easy to setup and administer, but offers rock-solid security and performance.

As it follows the XMPP based protocols, you can use any chat client that uses the same protocol. like jabber, Spark, Pandion, etc. i use Spark as my chat client. it's working realy amazing. got great functionalities. very fancy and attactive looks. one great feature is it's screenshot capturing and sending utitlity. try it and then feel how wonderfull it is.

you can see some openfire screenshots here.
and of spark screenshots here.

both the softwares are available for windows and linux operating system.