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.

freemp

A simple, lightweight WordPress plugin for displaying a notification bar at the top of the webpage if 1st-party JavaScript is disabled. If appropriate, the entire website may as well be blocked by a full-page overlay instead of just a small bar.
The plugin offers configuration of background and foreground color, opacity, notification message text and font size. Individual customization by means of CSS snippets is also supported.

JavaScript Notifier

JavaScript Notifier allows you to inform visitors that your website requires JavaScript.

WP Warning vs JavaScript Notifier