Tagxedo is, in my opinion, a souped-up version of Wordle. It takes words from pretty much anything (rss feeds, twitter, famous speeches, urls, etc) and turns them into word "clouds" with more frequently used words showing up larger.
Tagxedo is cool because you can change most of the options, including the shape of the final cloud! Here, I made a Tagxedo from the Sandburg Art Blog:
Aww, I made it a heart shape, because I love art! It is easy to save and print your Tagxedo without having to create any logins or users (which is really important to me when I'm working with students). This site was really fun to play around with, and I hope to use it with students very soon as part of a cool new project!