Sparanoid

Adds readability.com widget on your site. A must-have plugin if you’re a Readability publisher. This plugin also works fine with multisite enabled WordPress (aka. WordPress Mu).
More information please visit my <a href="http://sparanoid.com/work/readability/" rel="nofollow ugc">site</a>.

Readability Buttons (readability.com)

Adds readability.com widget on your site.

Mike Riethmuller

Better Writing is a WordPress plugin that helps you write better content.
Readability
One way it achieves this is by providing readability information as you type. You can select from one of 5 different readability scoring methods including:
<ul>
<li>Flesch reading ease formula</li>
<li>Coleman Liau index</li>
<li>Gunning fog index</li>
<li>SMOG Index</li>
<li>Automated readability index</li>
</ul>
You are able to see at a glance if your content is on target with the readability meta box on the WordPress edit screen. You can also set a custom target score and sort posts and pages by readability.
All features can be turned on or off for posts, pages or even custom post types.
Use details of your readability score elsewhere within WordPress by using shortcodes. 
* [readability-score] 
* [readability-method] 
* [readability-flesch] 
* [readability-coleman] 
* [readability-gunning] 
* [readability-smog] 
* [readability-ari]
Preferred terms
Consistent use of language is important and is something that can easily suffer, especially where multiple writers are involved. Better Writing allows you to maintain a list of your preferred terms and non preferred terms. It will prompt writers and editors with spell-check style suggestions in the content editor. It will check as you type and allows for multiple terms and multiple suggestions.
All features are designed to be non-intrusive and let you get on with your work, whilst giving you the tools you need to help you write better without distraction.
<h3>Credits</h3>
Credit to Dave Child for use of https://github.com/DaveChild/Text-Statistics.

Better Writing

Helps you write better content with readability scores and preferred terms as you type. Highly configurable.

Readability Buttons (readability.com) vs Better Writing