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.

Eric McNiece

EMC2 Popup Disclaimer uses Fancybox (http://fancybox.net) to display the popup, and jQuery-cookie (https://github.com/carhartl/jquery-cookie) to detect user session. Creates a cookie named “emc2pdc” that can be viewed with regular developer tools.
Easy to use! By default the script is added to the wp_footer action and will work quietly. You can also force the display on pages via shortcode or function call: 
Shortcode: <code>[emc2pdc id="42"]</code> 
 -or- 
Function: <code><?php emc2pdc_force( $atts=array(), $force=NULL, $id=NULL ); ?></code>
Check the demo at http://popup.emc2innovation.com !
To Do list:
<ul>
<li>Add No-JS fallback support</li>
<li>Create a selection of button themes</li>
<li>Add support for different lightbox types</li>
<li>Improve handling, add PHP cookie support</li>
</ul>
Known bugs:
<ul>
<li>Recursive post shortcode call results in chaos</li>
</ul>
<h3>Configuration</h3>
EMC2 Popup Disclaimer will work straight out of the box. There’s a few options available from within WordPress, and future releases will feature themeable options.
Settings can be found in these locations:
<ul>
<li>/wp-admin/options-general.php?page=emc2-popup-disclaimer/emc2pdc-admin.php (Default settings)</li>
<li>/wp-content/plugins/emc2-popup-disclaimer/js/emc2pdc.js (Fancybox, jQuery-cookie settings)</li>
</ul>
The settings page is straightforward except for perhaps the “Restrict operation to directory:” setting. This is the operational directory of the jQuery cookie plugin – default is ‘/’, which is your entire site. To specify operation for a certain directory, type it here like ‘/store’. More jQuery-cookie settings can be found at https://github.com/carhartl/jquery-cookie.
Things of note:
<ul>
<li>Specify post ID using shortcode is optional ? if blank, it will use ID# from settings</li>
<li>Shortcode / function call will override wp_footer action</li>
<li>Debug mode will output visible settings on every included page</li>
<li>Setting an ID equal to the page you?re calling from results in mass chaos! If you set the shortcode [ emc2pdc id=”10″ ] on post ID 10 it seems to freak out a bit.</li>
</ul>

EMC2 Popup Disclaimer

EMC2 Popup Disclaimer places a specified post into a lightbox and adds buttons so that your visitors must click through to agree to your terms!

WP Warning vs EMC2 Popup Disclaimer