Julian Yanover

Admin Msg Board is a plugin for WordPress that will add a messaging system in your WordPress admin area. This works for weblogs that are written by more than one blogger, so they can communicate with each other, without leaving the WP system. 
Messages can have up to 140 words like in twitter, and you can choose whether to write to all users or select recipients

Admin Msg Board

Adds a messaging system in your Wordpress admin area for bloggers to communicate.

richardphelps

This plugin allows users to chat in real time using AJAX. Users can connect with any other users.
Use the [bp-instant-chat] shortcode to embed the front end chat on any page.

BuddyPress Instant Chat

Instant chat plugin for BuddyPress allowing user to connect and talk in real time.

Admin Msg Board vs BuddyPress Instant Chat