One of the traps that many student writers fall into is overusing favorite phrases and adjectives. I've edited and graded enough essays over the years to confirm this. There are a couple of tools that can help students avoid overusing the same phrases and adjectives. 

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. 

Last spring at the Massachusetts School Library Association's conference Pam Bergerpresented the idea of using word clouds to help students analyze documents. Wordle is the "old reliable" of word cloud creation tools. Some other options for creating word clouds are Tagul, Tagxedo, and ABCya's Word Cloud Generator. 

Applications for Education 
Have your students run their text through one of these tools before they their papers to a classmate or teacher to read.