Kaspars

Metronet AS

Plugin adds a meta box to every post, page and custom post type where users with editor capabilities can promote other users (with author or contributor capabilities, by default) as editors of that post.
Plugin provides the following filters:
<ul>
<li><code>custom_editors_enable_roles</code> — Specify an array of user roles which should be allowed to be promoted as editors of specific posts and pages. The default is: <code>array( 'contributor', 'author' )</code>.</li>
</ul>
A plugin by <a href="http://metronet.no" rel="nofollow ugc">Metronet</a>.

Multiple Editors

Allow multiple editors and contributors per post or page. Also enable contributors and authors to upload files and add new pages by default.

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.

Multiple Editors vs WP Post Contributor