http://www.hemingwayapp.com/
It does basic editing for grammar, spelling, etc. but it also helps streamline awkward or overly long sentences, suggests alternates for some phrases, and warns you for other things like overuse of adverbs and stuff. If you don't have a proofreader or just want to improve your writing in general, it's definitely worth taking a look at.
That said, it's still a good idea to look it over yourself, since it's a computer doing the work and it isn't always 100% accurate.