Yes, there is such a plugin! Combine Word Statistics Plugin 1.0
with MS Word statistical and grammatical tools, double check with FireFox dictionaries and you’ve got yourself a handful of great safeguards against typos and common errors.

For those who don’t have access to MS Word, nor any other sophisticated word processor, they can always use an online tool such as Texalyser.net

Gunning-Fog and Flesch-Kincaid are readability indicators that are essential for those gearing content for specific audiences. For more information on readability indexes, you can consult each separate Wikipedia entries, but for the sake of brevity, here is a brief introduction.

In linguistics, the Gunning fog index is a test designed to measure the readability of a sample of English writing. The resulting number is an indication of the number of years of formal education that a person requires in order to easily understand the text on the first reading. That is, if a passage has a fog index of 12, it has the reading level of a U.S. high school senior. The test was developed by Robert Gunning, an American businessman, in 1952

The Flesch/Flesch–Kincaid Readability Tests are readability tests designed to indicate comprehension difficulty when reading a passage of contemporary (late 20th century?) academic (US?) English. There are two tests, the Flesch Reading Ease, and the Flesch–Kincaid Grade Level. Although they use the same core measures (word length and sentence length), they have different weighting factors, so the results of the two tests do not always correlate: a text with a higher score on the Reading Ease test over another text may have a lower score on the Grade Level test. Both systems were devised by Rudolf Flesch