Having tested virtually every Grammar/Spell Checker I can find, Serenity Editor (serenity-software.com/pages/home.html) leaves the competition standing. Frankly, it has no peers that I can find. Not only is it successful in correcting spelling and unseen grammar mistakes, it will also help in spotting cliches and tightening up lazy and passive writing. I wouldn’t finalise a manuscript without it. The drawback for Mac users is that it is Windows software. However, I’ve overcome that difficulty and it’s not too fiddly. I run Editor (the integrated Editor for Word version that puts an add-in into Word) through Codeweaver’s Crossover Mac software (you could also use Parallels or Fusion); installation is a little complex but doable even for a technophobe like me. The alternative is to use the standard version of Editor with Word and print off a list of errors.
Basically, I paste a scene or chapter from Scrivener into Microsoft Word (I use 2003) run Editor and underline potential alterations on the Word document as Editor goes through it. It’s then just a case of making alterations to Scrivener either by printing off the Word documents or having them side-by-side on the screen. A little fiddly but not bad once you get into the swing of it. And the results are wonderful! Tighter writing free of most errors.