I’ve heard of KISS Grammar for several years, but haven’t highlighted it because although all of the content is 100% free, and there are no ads, the KISS site is rather clunky and confusing. But, you know what? You might enjoy browsing a website to search out the gems, and gems there apparently are, as this free grammar curriculum is often suggested in homeschooling circles, particularly with those who prefer the Charlotte Mason style of educating.
So, here you are: free KISS Grammar Curriculum for homeschool (and schools!).