SenSEO is an extension checking the most important on-page-SEO-criteria and calculating a grade of how good your site fulfills these criteria.
The main goal allows users to open (and in some cases edit) page source with external applications.
Measurer allows to measure dimensions of the objects on html page. Press down shift for multiple measurements.
Good Website Inspector is an extended ruleset for YSLOW, a Firefox AddOn, to check source codes of HTML pages to get easy and fast hints to optimize the current page.
A suite of tools for developers.
Inspect flash(swf)'s DisplayObject with mouse. Overview the swf's DisplayObject list. Set the inspect filter class when use mouse inspect.
following @itamt on twitter for the updates
View xml data in textareas, in readable format.