I wouldn't think anyone has a grammar checker that's worth anything. One of my favorite games was taking one of those little things someone would hack together that had a whole list of homonyms that would make sense if the right words were there. "Eye sea ewe" for "I see you" for example, but more extensive. You feed it to a spell checker and it finds no problems. I would feed it to a grammar checker and find that it spotted very few problems.