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.

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.

Multiple Editors vs Contributors' Posts