This website allows users to add their own writing to the webpage and it will highlight sentences that are too complex and words that are unnecessary. It even gives you a readability grade and may offer some suggestions about how to improve the writing. (Watch the YouTube video below for a brief tutorial.) However, the suggestions aren't always correct and may mean a complete revision of the sentence if you follow the suggestion. Nonetheless, it has been a good tool to use with some of my struggling students so that they can see some of their errors. Mind you, we also talk about the fact that not all of the suggestions are correct (which happens with peer editors, too).
Hemingway is an easy website to use and it can be used on a PC, Mac or an iPad (though there isn't an app that I'm aware of, but you can see the site on the tablet). If you're looking for a way for students to start reviewing their writing, this would be a good tool. Despite my pleas for them to read their own work aloud, I'm sure not everyone does. Because Hemingway is a website that is so visual, I believe that after walking students through the process and explaining the possible defects, it will help highlight the issues present in student writing and encourage them to think deeply about what they are trying to say. Try it out, let me know what you think, and share your successes or ideas.