This is a free tool that I love. It is excellent for keeping your writing to the point and clear. You can use hemingwayapp.com for any type of writing, but I find it most helpful for blog posts and emails.
There is a paid desktop version available, but it features the same functionality. The only perk is that it can be used offline and you don't run the risk of losing content if the internet goes out our you navigate away from the site.