Markus Echterhoff

This plugin is as simple as it is useful. Add the widget or use the shortcode <code>[you-have-a-new-message]</code> anywhere to display a notice if the currently logged in user has new messages. The notice links to the user’s inbox as a convenient and omnipresent shortcut.
Message texts can be configured via widget interface and shortcode parameters, e.g. <code>[you-have-a-new-message single="new message" multiple="%s new messages"]</code>.
<h4>Translations:</h4>
<ul>
<li>German</li>
</ul>

You Have a New (BuddyPress) Message

Notify users about new BuddyPress messages (widget & shortcode)

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 …

You Have a New (BuddyPress) Message vs Site Notices WP