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.

Tigerton

WordPress plugin that tell visitors to turn of IE compatibility mode if enabled.
<pre><code>Check if IE compatibility mode is enabled.
Write a custom popup text.
Select on what page types you want to enable popup. Example: only on front page or on all pages.
Select if you want visitors to be able to disable the popup for future visits.
Debug mode, to allowe the popup be displayed always, in any browser.
</code></pre>
Stay tuned as it possibly will get updated with new features.

WP IE Compatibility Checker

Prompt users to turn off IE compatibility mode if enabled.

WP Warning vs WP IE Compatibility Checker