This bit is specifically for the writers and teachers on my friends list, but feel free to use this bit of recently-gained knowledge in whatever way you find most appropriate.
Open up a document in Microsoft Word (I hear this will work with other programs, but I only know how to guide you through Word).
Once it is open, go to Tools on the toolbar.
Select Spelling and Grammar...
When it indicates the first error it thinks you should fix, select Options... at the bottom of the box (NOTE: If you have no errors at all, make one intentionally so the program will find and try to fix it)
Put a checkmark in the box next to Show readability statistics
Finish the spelling and grammar check, and at the end you will get a rather thorough diagnostic of your writing, including words/characters/paragraphs/sentences counts, sentences per paragraph, words per sentence, characters per word, percentage of passive sentences, Flesch Reading Ease, and (this is the best part of all) the Flesch-Kincaid Grade Level at which you are writing.
You know, like sometimes you pick up a young adult novel that says it is for people ages 12 - 15 or grades 3 - 5 or whatever? Yeah, like that...
For example, what I have just written is at the 10.5 grade reading level.
Isn't that the coolest?!?
Go forth; have fun.