Chrome //
Extensions 
Chrome
Web store 
SEO Content Checker: Plagiarism & Readability
Easy-to-use SEO content checker, that helps to check text for plagiarism, readability, relevance, 'key'word stuffing.
Rocket Readability
This extension uses readability.js to parse out the 'cruft' leaving you with simply the article to read.
Readability Score
Given a body of text receive a score on how readable that text is based on the flesch-kincaid readability index.
boom! – Declutter pages, improve readability
Declutter webpages and improve readability... Point, click, boom!