FireScope is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS. The extension's core functionality is centered around a new Reference panel, which contains a search tool for looking up HTML elements, attributes, and CSS properties.
TYPO3.org is the main developer resource of the TYPO3 project, a free CMS framework released under the GPL. This site provides lots of information about creating sites, writing extensions and developing the core of TYPO3.
These bookmarklets let you see how a web page is coded without digging through the source, debug problems in web pages quickly, and experiment with CSS or JS without editing the actual page.