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. Note that Amazon.com's "Concordance" features allows something similar, but ONLY for books on its site that are searchable in their "Search Inside" feature. TagCrowd, created by Daniel Steinbock, a doctoral student in Design and Education at Stanford University, allows you to do this with any text. You can read more about TagCrowd, and some interesting ways to use it, TagCrowd: joining the crowd together.
Here is an example which will let you find the 100 most frequently appearing words in the Mark Twain classic, Tom Sawyer, the digitized text of which is available from the University of Virginia.