It looks like this has been up since last summer, but today was the first I have heard of TagCrowd. TagCrowd is a web based tool that allows you to visualize word frequencies as a tag cloud. You can visualize web pages, text files, and contents of your clipboard and then embed the tag cloud in your page. This could be great for teachers to use to visualize frequently used vocabulary from text sources. Amazon has a similar feature, concordance, for books that are searchable in their Search Inside feature, but TagCrowd allows you to do this with any text. The example below is from the freely available (in public domain) text of Tom Sawyer by Mark Twain at the University of Virginia.
TagCrowd was created by Daniel Steinbock, a doctoral student in Design and Education at Stanford University. You can read more about TagCrowd, and some interesting uses ideas, on the TagCrowd Blog...
By way of NancyW and Twitter...
TagCrowd Example: The 100 most frequently found words in Tom Sawyer available at the University of Virginia...