A cross-browser bookmarklet to examine the box-model of any DOM element on a page. Runs on Internet Explorer, Safari (and Webkit based browsers), Mozilla (including Firefox and Camino).
A DHTML shell that allows you to query the current browser window. Includes command line JavaScript execution, mouseover DOM query, CSS input, tab completion, profiler and object inspection. Can be configured to run off a bookmarklet, and works in IE, Firefox, Safari and Opera
Mark Pilgrim's essential online book about developing Greasemonkey scripts. Excellent starting point for Greasemonkey developers. Covers everything from setting up a Greasemonkey scripts to common DOM coding idioms, as well as a brief rundown of XPath, and case studies of Greasemonkey scripts.
Mark Pilgrim dissects the security issues of the 0.3 Greasemonkey, and describes how the new architecture of Greasemonkey works, pointing out pitfalls and how to address them.