AS OF MAY 2016, THIS PLUGIN IS NOT BEING MAINTAINED AND WILL NOT RECEIVE ANY FUTURE UPDATES. 
CONTACT ME IF YOU’D LIKE TO LIST AN ALTERNATIVE HERE.
Shows word and sentence counts plus a readability analysis of the post 
currently being edited using three different readability measurements.
Readability analysis is an attempt to show how difficult a text is to read. 
There are several methods of doing readability analysis. The most popular 
methods are used here. The Flesch and Flesch-Kincaid methods use formulas based 
on the average number of words per sentence and the average number of syllables 
per word. The Gunning-Fog method uses a formula based on the average number of 
words per sentence and the percentage of “hard” words (words with 3 or more 
syllables) in the passage.
Both the Gunning-Fog and the Flesch-Kincaid give a score that is the number of 
years of education the reader must have completed to understand the text. 
Different types of writing calls for different scores. Technical subjects may 
require a higher level of education so a higher Fog or Kincaid score is 
acceptable. Informal writing may try for a score in the 7-10 range. The Flesch 
score is a percentage. The higher the value, the better the readability.

FD Word Statistics Plugin

Shows word and sentence counts plus a readability analysis of the post currently being edited using three different readability measurements.

Scribit

Proofreading plugin allows you to improve the quality of your posts, pages and all your WordPress website. 
It gives you the possibility to check the correction of the texts inserted into posts, pages and drafts in less than a second!
Press the “Analyze” button and you will have a list of all found errors. 
The plugin allows to recognize different types of text errors (thanks to Languagetool.org). Here some examples of error types: 
* Typos 
* Punctuation 
* Typography 
* Commonly Confused Words 
* Nonstandard phrases 
* Semantic 
* Grammar 
* Redundant Phrases
Proofreading plugin supports a lot of languages. For example: 
* English 
* Spanish 
* German 
* Italian 
* Japanese 
* French 
* Portuguese 
* Russian 
* and so on…
For every language, in the settings page, you can customize which are the error categories you are interesting, hiding the others.

Proofreading

I think it is very useful inside Wordpress world. All site must have it to avoid common text errors.
It does not take up much space in the backend Wordpress console so it can always be left activated.

P.S: I'm the autor of the plugin.

Very useful

Proofreading checks texts on every content of your WordPress site. You can proofread in 30 different languages and get tips to improve your writings.

FD Word Statistics Plugin vs Proofreading