A teacher mentioned her frustration that google doesn't seem to catch all the grammar and spelling mistakes that students make, so I've been investigating options, and I've come across one called Grammarly which can be added as an app to your google chrome account. To my surprise, it also shows up when I write emails and other online documents as well! You know it's active when you see the little spinning G at the bottom of your document. Misspellings or grammar errors are underlined, and when you hover over the word, a pop-up screen appears to ask you about whether you meant something else instead - so polite! It may even give you a little explanation. Grammarly just sent me an email today reminding me of some of its other features. The one I especially liked was that you can double click on a word which gives you a definition, pronunciation, and synonyms. If anyone tries it, let me know how you like it! There is a premium version which you pay for, but I haven't really felt a need to upgrade - the basic version has sufficed.
Addendum: Shortly after I published this, I ran into another very positive review - Website Review: Grammarly