YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
Finds HTML elements matching chosen CSS selector(s) or XPath expression
Pixel Perfect is a Firefox/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML.
Inline Code Finder is an add-on to Firebug, to be able to find HTML elements with any of the below issues:
* Inline style
Detailed description can be found athttp://www.robertnyman.com/inli...
SenSEO is an extension checking the most important on-page-SEO-criteria and calculating a grade of how good your site fulfills these criteria.