Comment tools for Essay grading
You don't need an add-on to create custom comments in MS Word. Go to Tools on the menu bar and open up Auto Correct. Create custom comments here. Try it out: in the text field on the left type "cs" and in the text field on the right type: "You have committed a comma splice in this sentence. A comma splice occurs when two separate independent sentences are joined with only a comma." Save it. Now any time in MS Word you type "cs" and then hit the space bar those two sentences will appear. I have used Auto Correct to add all sorts of custom comments on punctuation, spelling, grammar, topic sentences, paragraph structure, and the like.
If you are grading essay submitted as pdf files, there is also a tip from Adobe for adding comment stamps, see http://www.adobe.com/education/acrobat/tips/acrobat_stamps.html
Is there a way of easily adding comment for work submitted as web pages?