Alsvin Tech

This plugin allows you to display different kind of notifications to your site users, or show special information or alert messages on any specific page/post/product.
Features:
<ul>
<li>Display site wide notices</li>
<li>Display notices on any specific page/post/product</li>
<li>Choose from any four pre-defined color schemes/notification types</li>
<li>Extremely customizable, set your own color, icon, title and message</li>
<li>Display notices on six screen positions</li>
<li>Two animation styles</li>
<li>Instantly preview notices, to see how it looks on front-end</li>
<li>Easily customizable through CSS</li>
</ul>

Site Notices WP

Show notices, alert, popup, info messages on your WordPress site easily. Choose where to display notice on any single page/post/product or on complete …

Jenst

Cookie Message plugin will on activation add a message at the bottom of your screen.
When clicking the button it will disapear. How long is for you to decide but default is forever. It will remember it by HTML5 localstorage, not by a cookie.
You have settings for information that goes into the message, colors, fonts and you can disable CSS and javascript if you need to. See screenshots for more information about the settings.

Cookie Message

EU cookie law message at the bottom of the screen.

Site Notices WP vs Cookie Message