[url=https://wordcounter.net/]Word Counter[/url]: I use this to keep track of how long things are. For RP's I prefer to make posts 2k words in length, so I use this to see where I am landing. It mostly just keeps me paying attention to wordcount. It does have a lot of other useful stats though, and my second favorite feature is keeping track of most commonly used words. I use that to make sure I am not flooding my post with any specific noun.

[url=http://www.hemingwayapp.com/]Hemingway App[/url]: Mentioned in the OP of course, but I got into using this one recently. I don't necessarily take it as gospel, but it is good for showing you what parts you should reconsider. 

I also use a text to voice add-on for Firefox. It isn't perfect, and it is especially bad with names, but I think having text read out to you not only helps you catch errors, but is also good for double checking dialogue.

Weirdly enough, I actually prefer writing posts in PMs to writing them in word processors. I never particularly liked Google Docs, and though I have Scrivener for organizing my many half-baked attempts at serious writing, I don't like writing RP posts in it. I have a specific self PM I use for storing notes and writing drafts, since my style nowadays is to write a little bit at a time.