Full Screen Slide Show for paginated documents, such as PDF, DVI, and Xdoc.
Numerous behaviors do interesting things with the selection. Clicking the alternative mouse button over the selection pops up a menu of other actions. The selection can be sent to another web server. It can be looked up in Encyclopedia Britannica, defined by Merriam-Webster (try it with "multivalent"), sent to a web search engine or USENET newsgroups, or decoded by the acronym server. By following the pattern in the relevant XML hub, the list could be easily extend to include language translation and movie database.
Another behavior that happens to be bundled with the base system, and which was written in two hours after I found some sound files, will take the numbers and letters in the selection and generate the corresponding telephone touch tone sound. Try the following 3212333,222,133,212333322321. (Note that Windoze can't mix sounds from multiple sources, unlike BeOS on the same hardware, so if some program such as an MP3 player already claimed the sound card, you have to turn it off before you can hear the touch tones.)
A general table sorting behavior can sort the columns of CALS tables, which are used by HTML. Few HTML tables actually have tabular data—they are used almost exclusively for layout—but the media adaptors for local directories, Zip, and other listing/archive types do. Columns are inspected to determine the data type (string, integer, or floating point; dates too by clever media adaptors), then sorted by that type; sorting twice on a the same column sorts it in reverse order.
As described above, a number of annotation types are made by selecting some text choosing an annotation type from a menu, and perhaps entering supporting information. After making a number of copy editor marks and other annotations, one can obtain a concise "executive summary" of all markup by choosing View/Executive Summary. The page is collapsed to show only structure (such as HTML H1 tags) and the annotation groups selected as View/Annotations as Notemarks and View/Copy Ed as Notemarks.
Another type of visualization for search results was independently developed by TkMan and Donald Byrd of University of Massachusetts, Amherst (see "A Scrollbar-based Visualization for Document Navigation" in ACM Digital Library Conference '9x).
I like a feature of Tk's text widget: clicking button 2 anywhere in the text and dragging scrolls—you don't have to move over to the scrollbar.