PaperRater is a free service that reports on the quality of your writing. Paste your text into the page and in just a few seconds, you get a detailed review in a half dozen categories. Not only does the site do standard spell and grammar checking, but you get an assessment if the content has been plagiarized, details on word choice, word usage, and the complexity of your word choice.
You can correct spelling and grammar problems right on the page -- problem words are underlined, much like in Word. There's also a lot of supplemental information to help you understand the site's analysis. And the plagiarism checker is a handy reminder to properly cite your sources. It correctly identified some articles I had previously published online as "possibly plagiarized."
This is the best writing help you're likely to get short of paying a professional editor to review you stuff. [via Download Squad]