eisenivan

A lightweight plugin that allows you to place a sitewide alert message in the header of your site. There are options to include a default set of styles (based on Twitter Bootstrap), center your alert, and switch the style class.

Summits Alert

A lightweight plugin that allows you to place a sitewide alert message in the header of your site.

Brian Messenlehner

The WP-Announcements plugin is ideal for websites or blogs that would like to display a site wide announcement, featured post, important message, community notice, advertisement, survey form, email subscription form, emergency alert or anything you can publish in a post as a marquee or popup window using ShadowBox-JS or ThickBox.
WP-Announcements provides two announcement options:
<ol>
<li>
A customizable marquee that displays an assigned post title linked to that actual post. The marquee can be displayed by calling the php function wp_announce_marquee(”) anywhere in your theme, using the WP-Announce-Marquee Widget or using the Shortcode [wp_announce_marquee] anywhere in your post or page content.
</li>
<li>
A popup window using ShadowBox-JS or ThickBox that displays the title and content of an assigned announcement post. The popup will only show up once for each website visitor per active popup post as a cookie is created. You can also set a popup window for each time a website member logs into the admin dashboard.
</li>
</ol>
WP-Announcements activates checkboxes on the add/edit post page to allow easy assignment of announcement posts. An admin page under the settings tab is also included to toggle announcements on or off and to manage all posts saved as announcements for easy retrieval and reuse of reoccurring announcements in the future.
Works with WordPress and WordPress MU 2.8.1
For more information check out:
<a href="http://webdevstudios.com/support/wordpress-plugins/wp-announcements" rel="nofollow ugc"></a>WP-Annoucements Page
<a href="http://webdevstudios.com/blog/wp-announcements-wordpress-announcement-plugin" rel="nofollow ugc"></a>WP-Annoucements Post
<a href="http://webdevstudios.com/support/forum/wp-announcements" rel="nofollow ugc"></a>WP-Announcements Support Forum
<h3>Plugin Support</h3>
<a href="http://webdevstudios.com/support/forum/wp-announcements/" title="WordPress Plugins and Support Forum" rel="nofollow ugc">WP-Announcements Support Forum</a>

WP-Announcements

Display a post as a site wide announcement, notice, ad or emergency alert on your website via a marquee or popup window using ShadowBox-JS or ThickBox …

Summits Alert vs WP-Announcements