Very similar to Wordle but in many ways I think a lot better for use in school.
First, and I think most importantly, you can save your cloud as an image file. There are more options for personalising your cloud, choosing the shapes, colours, fonts, etc. Just like Wordle, you can upload a document or use a url to generate your cloud. If you haven't used a program like this before what it does it generate your cloud based on the words in your document/webpage, the more frequently words appear the larger they will be.
I like to use this to when children have done a piece of writing. After typing it up they can upload it to the site and see which words they have used most frequently. It's a really good way for them to spot words they have overused and think more carefully about word choices. Also they make really effective artwork for displays!