If you like using Wordle, then you'll also LOVE Tagxedo!
The basic idea is the same - you can turn words into a word cloud. Tagxedo also lets you pick different shapes, outlines, etc. that the words can form in and around.
Here is an example for this blog iHeartLiteracy. The words generated are all from how frequently they occur on my blog, and I picked the heart because... well... I just heart literacy!
Your students will enjoy playing around with this site. The combination of shape and words can play off of each other in interesting ways to help create meaning for the image.