Hemingway is a free tool designed to help you analyze your writing. Hemingway offers a bunch of information about the passage you've written or copied and pasted into the site. Hemingway highlights the parts of your writing that use passive voice, adverbs, and overly complex sentences. All of those factors are accounted for in generating a general readability score for your passage. A new beta version (you can opt-into it) includes tools for formatting your text within the Hemingway editor.
WordCounter is a simple tool that writers can use to identify the words that they use most frequently in their text. To use WordCounter simply copy and paste text into Wordcounter then select how many words should appear in your "frequently used" list. To improve the utility of your "frequently used words" list you can tell Wordcounter to ignore small words (like it or the) and to use only root words.
StoryToolz offers a few tools to help you edit your work. The Cliché Buster analyzes your work to find clichés that you have used in your writing. The Readability tool analyzes your text to estimate a reading level on several scales.
Word clouds can help students analyze their own writing by showing them the words that they use with the most frequency in their works. Wordle is the "old reliable" of word cloud creation tools. There is a Google Docs Add-on called Tag Cloud Generator that will create a word cloud within a Google Document. Some other options for creating word clouds are Tagul, Tagxedo, and ABCya's Word Cloud Generator.