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.

crissoca

Alert generator plugin. Generates a slick alert bar (info, warning, success, error or note).
Use it by inserting a simple shortcode, or directly into the template files by using:
<pre><code><?php echo do_shortcode('[alert type="type"]Type here![/alert]'); ?>
</code></pre>
Alert types:
<pre><code>[alert type="info"]Info here![/alert]
[alert type="success"]Success here![/alert]
[alert type="warning"]Warning here![/alert]
[alert type="error"]Error here![/alert]
[alert type="note"]Note here![/alert]
</code></pre>

Alert Me!

Alert generator plugin. Generates a slick alert bar (info, warning, success, error or note).

WP Warning vs Alert Me!