genkisan

<ul>
<li>easily switch on and off through Admin Options</li>
<li>display message without editing theme files (above 1st post or through widget)</li>
<li>display different message to different roles/users</li>
</ul>
Demo: 
<a href="http://ericulous.com/2007/03/09/genki_announcement/" rel="nofollow ugc">Plugin Details & Demo</a>

Genki Announcement

Magicianred

This plugin is useful for those who want to add to your blog Columnists or other Characters without having to create new WordPress Users, you can associate these contributors also to pages and posts.
It can also be useful to writers who wish to create posts that relate to the characters in their stories, or as if they were the characters of their own stories to write them.
With this plugin you can add Contributors’ Posts, Contributors and, through shortcode, add content to posts, or through widgets, add to the sidebar lists or details of contributors and their posts.
Plugin localized with PoEdit. 
Localization made: Italian (me).
Follow the updates of the plugin subscribing to http://simone.paolucci.name/wordpress/plugins/contributorsposts/contributorsposts.rss.php
<h3>ToDo Section</h3>
Nothing to do at the moment. I accept your suggestions.

Contributors' Posts

Contributors' Posts is a plugin to manage posts of contributors with the same account without having to create other users. For online Press.

Genki Announcement vs Contributors' Posts