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.

mrsajith

You can set your message /news here.You visitors can now see your announcement. The Plugin is created with light window support. The animated effect on announcement will give a pleasant effect for your blog/website. It will be displayed only one time for a particular user . The plugin is created with cute design and animation. It will never interrupt your blog reader.
Features:
<ul>
<li>Navigation: You can navigate your reader into a particular event</li>
<li>Message Passing: You can announce any event information here</li>
<li>Html content: You can add images, links, color just like a post (RTE). </li>
</ul>
For Example and Details Visit : http://www.sajithmr.com/wordpress-announcement-plugin/
My Technical Blog is : http://www.sajithmr.com/
<h3>Arbitrary section</h3>
If you have any queires / complaints regarding this plugin, please feel free to mail me at admin@sajithmr.com. Thank you
See my blog for technical updates <a href="http://www.sajithmr.com/" title="Technical Digest" rel="nofollow ugc">http://www.sajithmr.com</a> .
<blockquote>
Use announcement and improve your website quality. Happy Blogging
</blockquote>

annouoncement

Do you wanna inform any important news or message to your readers ?  You can use this plugin.

WP Warning vs annouoncement