The first way is to highlight the most frequently misspelled words for the week (see below). All of the students would type in the word that they missed on the spelling test (spelled correctly, of course) and the words missed the most would appear the largest. This could be printed out and hung up in the classroom, or maybe posted on the classroom blog for the students and parents to review.
Another fantastic way to use Wordles is to make creative posters that don't necessarily relate to the lesson plan, but showcase the class itself. One idea would be to make a Wordle of everyone in the class (see below). Each student would write his or her name into the text box and it would make a cool project for Open House or a Parent-Teacher night.
Finally, I think Wordles could be useful when summarizing stories or large blocks of information. Instead of making my students read a 9 page web page, I just paste all of the text into the Wordle generator and have a visual representation of the main words. I made a Wordle (below) for the SparkNotes webpage about Western Expansion and I think that this could help students comprehend the main points.