Help Teaching, offers both free and paid services, for all courses offered in a school. So far, I have only used this website for Grammar and Language Arts. It is a nice quality site.
Cengage offers free online grammar quizzes. I have not actually used this site yet, but will be being used in the very near future.
At one point, I had typed into my search browser, fun ways to learn Language Arts. I mean lets face it, it is pretty boring, unless you love Grammar rules, bleh. Well one of the websites that came up was, Many Things. org. I have not tried this website yet, but my 4th grader will begin using it here at the end of the year to review.