Remi Corson

This plugin helps admins to easily create admin notification inside the WordPress administration.
You can choose many options and decide how and where to display the notifications.

Easy Admin Notification

Tested up to 3.3.1 Stable Tag: 1.4 Adds the ability to create easily notification in the admin panel

Re4DeR

This plugin is very usefull if you want warn people going to one category.
It set cookie, co visitor will see alert message only once.
For example you have a magazine and in one category you have adult content. 
With this plugin you can set up warning for access to this category-and user have to confirm your message.
<h3>A brief Markdown Example</h3>
Ordered list:
<ol>
<li>Choose between alert and confirm</li>
</ol>

WP Warning

This plugin allow you warn visitors when they want to go in category.

Easy Admin Notification vs WP Warning