Grammarly is well worth the low cost for the complete version. It is usually accurate in performing the daunting task of finding and correcting grammatical errors. Its better than most. I use it all the time.

My trusted Frnd when it comes to grammar correction.

Sometime the extension don't detect the text field, also sometimes it asked me to login when I already logged in!

The app keeps cutting out and I have to keep installing it every few days. VERY inconvienent (like now-it is apparently off AGAIN. Their support team was not very helpful as you can see.