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 …

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

Site Notices WP vs Banner Alerts