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

sujitkumar87

WP Post Contributors plugin allows you to add more than one author to the post who have contributed. 
The contributors meta will apear just aside of your post body. Just check the authors who have contibuted. 
And update the post to save it.
Features of Custom News Widget:
<ul>
<li>User friendly and easy to use.</li>
<li>Tested with other popular plugins.</li>
<li>Add authors to the post easily.</li>
</ul>

WP Post Contributor

WP Post Contributors plugin allows you to add more than one author to the post who have contributed.

Genki Announcement vs WP Post Contributor