UPDATE #2 I previously blogged a post about Grammarly - a Grammar Checker plug-in for Word, on 15 February 2015 when I mentioned that I had uninstalled this Grammar Checking app after personal bad experiences and poor reviews elsewhere. link
However, they seem to have updated the previous version and the Word plug-in is now FREE, it does not enjoy as many functions as the paid-for option. I used it to proof my Ph.D. Thesis, 60k words and it nearly made it to the end. There seems to be a limit on how many edits it will allow in the semi-automated mode. I finished the job of manually but I suppose it did about 50K words before that happened. I have not looked into the issue at this stage because I don't have the time. But it did pick up some interesting stuff but it is slightly more dyslexic in some areas than I am. But that was OK because it was consistent and I just ignored the suggestions. Would I pay for it? No, I don't think so but if you can cope with the minor limitations of the free version I would suggest it is worth a try. There is also a free plug-in for Chrome which I am trying.
If you want to check out the reviews I suggest you Google them, there are too many to list here.