The best grammar and spell check on the web by far Rated 5 out of 5 stars
It works and I use it everyday.
Why I highly recommend this:
If you have ever read any books such as "On Writing Well" by William Zinsser or "Elements of Style" by Strunk this plugin has those types of grammar ideas built into it! It warns you about things like the passive voice and other common mistakes in writing that a simple spell check would not.
I hope they continue to add develop this great plugin. I use it mostly for blogging and leaving comments.