Openfire

Openfire is a free, open source XMPP server.  It also happens to be the best XMPP server for small-medium business, and has clustering support for very large installations.

It can be connected with Asterisk to provide information about who in your organisation is on the phone.  You can also set it to log all chats and enforce chat rules.

One of the best features is the ability to set up an online chat system with your customers.  You can take questions from your website and answer them in your IM client.

Very handy!

http://www.igniterealtime.org/projects/openfire/