kengher

Broadcast simple, important alerts across your multisite network. Also great for single sites.
Features of Site Alert: 
* Display alert(s) in the Homepage with option to show in all pages of a site. 
* Native WordPress WYSIWYG message creation box for simplicity with media support. 
* Choice of hardcoded at the different parts of the page, or make it a pop-up modal. 
* Indicate different priority levels (colors) and icons. 
* Set a date range to publish/unpublish the alert.
Features as a Network Admin 
* Show alert across all all sites in your multisite network. 
* Exclude alert(s) from certain sites. 
* Site Groups manager to provide easier bulk exclusions.

Super Simple Site Alert

Broadcast simple, important alerts across your multisite network. Also great for single sites.

Adam Walter

Admin Notice is a very simple plugin that allows you to display custom notices in the WordPress admin that is visible to all users.
Control over the notice text and styling is available from the plugin’s settings page.
Developers can use the <code>admin_notice_enable</code> hook to control the notice’s visibility on a more granular level. The following example demonstrates how to show the notice only in the “Posts” area of the admin.
<pre><code>add_filter('admin_notice_enable', function($enabled) {

 $screen = get_current_screen();

 if (isset($screen->post_type) && $screen->post_type !== 'post') {
 $enabled = false;
 }

 return $enabled;

}, 10, 1);
</code></pre>

Admin Notice

Nice Plugin!

Display a custom notice to all users in the WordPress admin.

Super Simple Site Alert vs Admin Notice