GreaseMonkey

FireFox (Mozilla?) plug-in that acts kinda like a Web Proxy built into the browser, to muck with the content of pages you view.

http://greasemonkey.mozdev.org/

Mark Pilgrim ebook http://diveintogreasemonkey.org/

• http://diveintogreasemonkey.org/install/what-is-greasemonkey.html

uses JavaScript

fragile like a Screen Scraper