Another site that helps students get a sense of their own writing is "PaperRater".
From the website:
"PaperRater.com is a free resource, developed and maintained by linguistics professionals and graduate students. It is used by schools and universities in over 46 countries to help students improve their writing."
PaperRater will even provide the following services:
- Plagiarism Detection
- Auto Grader
- Spelling and Grammar Check
- Style and Word Choice Analysis
- Readability Statistics
(In an article for SLJ, Krista Brakhage speaks about how she uses it herself! LINK)