AjaxTrans does an Ajax-y realtime translation of text in one box into a second box in the language of your choice. Of course, while technically heroic and watercooler-cool, the actual real-person application of the thing escapes me.
A re-spin I would like, however, is to turn the idea into a Greasemonkey script. Wouldn’t it be absurdly useful if, in-browser, you could highlight a line of text on a page and have it be translated in place from Language A to Language B? Yowza.