Peter Westwood

This plugin allows your users to configure a reminder to be sent to them if they haven’t posted in the last n days.
You configure the reminder on your Profile page (or the profile of another user if you have than capability)
It is in the Personal Options section and called Blogminder Threshold

PJW Blogminder

This plugin allows your users to configure a reminder to be sent to them if they haven't posted in the last n days.

Pratik Panchal

Admin Notebook is simply allow wordpress site admin to add notes. 
Your added Note will save automatically & Display on Front & admin side all pages when you Logged in.Its resizable & Movable to any where in active Screen.

Admin Notebook

New Admin Notebook is simply allow wordpress site admin to add notes.

PJW Blogminder vs Admin Notebook