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.

Valice

Banner alerts are custom alerts to notify your website visitors of promotions, alerts, events, etc. These display at the top of the website until they are dismissed by the visitor. Any new visitor who has not been to the website will see these until they are dismissed.
Banner alerts are configured just like a post or page with support for title, content, and excerpt. The plugin settings allow you to define which parts of the alert to display and whether or not to link to the full post. You can run more than one alert at a time and activate or deactivate as needed.
This plugin is ideal for:
<ul>
<li>Important operational changes</li>
<li>Technical notices</li>
<li>Time-sensitive appeals for donation or action</li>
<li>Terms or agreement changes</li>
<li>Privacy notice acknowledgements</li>
<li>Service outages or maintenance messages</li>
<li>Special offers or promotions</li>
</ul>

Banner Alerts

Provides an easy interface for creating and displaying alerts or notices as a banner on a website

Cookie Message vs Banner Alerts