From the website description:
Wordle is a toy for generating "word clouds" from text that you provide. The clouds give greater prominence to words that appear more frequently in the source text. You can tweak your clouds with different fonts, layouts, and color schemes. The images you create with Wordle are yours to use however you like. You can print them out, or save them to the Wordle gallery to share with your friends.
Writers should strive to reduce repetition in their word choices, especially bland or less-descriptive words. Wordle is a tool that helps do just that by showing at a glance which words are overused.
Copy and paste a segment of text into Wordle's "create" box, and then it'll generate a word cloud like the one above (made from the first 20 pages of my current manuscript). The bigger words are the ones used most frequently. From the cloud, I can easily see I might want to reduce my usage of "note", "one", "like", and "looked" in those first 20 pages.
Wordle has options to change the font, orientation, and color of your cloud:
It also allows you to turn off the "remove common words" option (warning--not for the faint of heart, especially if you're writing in first person!):
But this one is helpful too--I can see that I need to look at replacing passive verbs like "was" with more active ones.
The website isn't perfect--unfortunately there's no way to export your clouds at this time, so I had to take screenshots and crop to size for the ones above--but it can be a valuable tool for writers.
Plus, those word clouds look pretty awesome.