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.

SalvatoreDP

WP Readability Analysis is a wordpress plugin that calculates the index 
of readability of a post or page using various methods: Gulpease, Gunning-Fog, 
Flesch and Flesch-Kincaid. 
With these formulas we try to analyze how difficult it is to read a text.
The method Gulpease, calibrated for the Italian language, unlike the other 
is based on word length in letters rather than syllables.
Methods Flesch-Kincaid and Flesch, who are the most famous, using formulas based 
on the average number of words per sentence and the average number of syllables per word.
The method Gunning-Fog uses a formula based on the average number of words per 
sentence and the percentage of words with 3 or more syllables.
Unlike Gulpease, these three methods are mainly used in the English language. 
Both Gunning-Fog that Flesch-Kincaid return a score which is the number of 
years of education that the user must have to understand the text.
I invite you to know more about the various pages of wikipedia devoted 
to the various formulas.

WP Readability Analysis

Shows readability analysis of the post or page using Gulpease, Gunning-Fog, Flesch and Flesch-Kincaid readability measurements.

Readability Buttons (readability.com) vs WP Readability Analysis