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.

tas2580

This Plugin calculates the Flesch readability index for the content of an article and displays it at the bottom of the article.
For more information about the Flesch readability index see: https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_tests

tas2580 Readability Index

Calculate and display the Flesch-Reading-Ease-Score of an article.

Readability Buttons (readability.com) vs tas2580 Readability Index