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.

Lucas Garcia

Posts reminder notifies you when you forget to write a post on the current day. It notifies you at 22:00 to the email you specifiy on the options page.

Posts reminder

Posts reminder notifies you when you forget to write a post on the current day.

PJW Blogminder vs Posts reminder