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.

Tim van Iersel

Change the browser tab Title when the tab is not active as a reminder or to get the attention back from the user.
The variables that can be set are:
<ul>
<li>Time before the title changes</li>
<li>The title when page is inactive</li>
</ul>

Browser Tab Title Reminder

Change the browser tab Title when the tab is not active as a reminder or to get the attention back from the user.

PJW Blogminder vs Browser Tab Title Reminder