I didn't find anything free even on sourceforge. I used a free one I'm sure a few years ago. My tendency is to use clauses instead of simple Hemingway type declarative sentences. Like, this idea connected to something, but, this other idea. I think many of the checkers just look for words "but" and "and" and stick a period after the preceding word, capitalize the following word, delete the "and" or 'but" then insert 2 spaces in its place.
Thing is if I write short sentences like. "He did this. He then did that." I feel like I'm writing "see Jane and Dick kick Spot's ass" or something.
When trying to find an easier hotkey every choice seems to stick " " in the text. Guess I'm stuck with Control-Shift-s.