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.

luzifer

With this plugin you get a new widget on your dashboard showing you every post on which 
you haven’t commented as last person. This is very usefull for blogger who wants to give 
the attention to their commenting readers they should have.
So if you try to reply on every comment because its good behaviour in your sight this plugin 
will help you not to miss one of them.

WP-Show-Unresponded-Comments

This plugin adds a widget to your dashboard showing every post which has a comment…

Admin Msg Board vs WP-Show-Unresponded-Comments