lunarmobiscuit

Per Post Editors is a plugin that adds to the WordPress permissions models, allowing you to assign specific users to be editors 
of specific posts or pages.
To summarize:
<ul>
<li>Allow specific users to edit a post or page</li>
<li>Editors are assigned directly inside the post itself – there are no other admin settings</li>
</ul>
<h3>Documentation</h3>
Full documentation can be found on the <a href="http://lunarmobiscuit.com/plugins/per-post-editors/" rel="nofollow ugc">User Permissions Page</a> page.

Per Post Editors

This plugin provides the ability to configure editors per-post.

keegnotrub

An update on the incredible Art Direction plugin by Noël Jackson (http://noel.io) and Dave Rupert (http://daverupert.com). Allows you to enter custom CSS to be applied per post/page.
Additional features include:
<ul>
<li>Optionally place span classes on your post/page title characters </li>
<li>Optionally place span classes on your post/page title words</li>
<li>Ability to responsivley use viewport units on post/page title</li>
</ul>

Artsy

Art Direction for WordPress - Allows you to enter custom CSS to be applied per post/page.

Per Post Editors vs Artsy