This plugin will add a dropdown from your address bar where you can quickly scan the HTML5 document outline, a must have for HTML5 developers.
Another must-have, Speed Tracer will help you debug performance problem in your sites. It can help you located where slowdowns occur, and hopefully help you fix them. It helps that Speed Tracer is very visual, giving you most of the information in a clean, well thought out manner.
Pendule is a very useful extension to the default Chrome developer tools. Definitely give this one a shot.
Search WordPress.org Codex
This is a simple plugin that will allow you to quickly search the WordPress codex by typing ‘wp’ into the address bar. Quick, easy and efficient way to look up anything wordpress.
A useful plugin with a funny name – Chrome Sniffer will discover the CMS of any website, from WordPress to Joomla to Drupal – in fact, it can discover up to 100 different platforms. A great tool for the curious!
PHP documentation – PHP.net
In the same vein as Search WordPress Codex, this plugin will help you learn more about those esoteric php functions with a handy search tool.