For my formal editing in Word and other word processing programs I use Grammarly primarily. One thing I love about it is the "plagiarism" feature. While I do not plagiarize source materials, it does allow me to find other sources that have used similar phrasing about the same subject. In one case, I actually found a reference that pointed out that a source that I intended to use as a resource had actually plagiarized much of what I was about to quote. While I still used much of it, I was able to redirect my footnotes to the earlier and proper source.
For online and website editing I use After the Deadline, an often free addon to Chrome and Firefox. If set to automatically find errors or make suggestions, it helps me edit while I am writing and before I publish. It's not perfect (I am always going back and finding errors in my published writing) - but it does help me correct most of the errors as I go along.
I recently read an article about a local Protestant (read "holy roller") independent church school that decided to toss out the state approved English grammar text book because it contained too many phrases and words the pastor did not approve. When the local newspaper reviewed the text book they planned to use instead, it ran a PDF version through its own spelling and grammar checker. The result was that over 50% of the paragraphs in the book (not examples) had moderately serious spelling and grammar errors.
The biggest issue that most of us have is that we are either too lax about spelling and grammar or we become anal and go overboard, checking everything we send out (Facebook, email, notes to family) to make sure that they are absolutely correct. When we do that we often take out the fun and personality inherent in our own writing and turn it into something that sounds like it was written for Funk & Wagnalls...
JV