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.

Michele Albert

Ultimate Slack Notifications

Notify your team members with each WordPress site activities by sending customized messages into Slack channels

Admin Msg Board vs Ultimate Slack Notifications