Here is a list of a few Google Chrome extensions useful for testers
Chrome link checker >>>
This extension will allow you to detect quickly invalid links inside a web page.
Flash block >>>
This will allows you to detect where is the flash content on a page as soon as you load it. This extension will also helps you to lower the memory usage.
IE tab >>>
With IE tab classic you can run Internet Explorer in Google Chrome.
Chrome pad >>>
Chrome pad is great if you need to write a bug while you are looking at your browser window. Instead of jumping between you bug tracker tab and the page you are testing, you can keep the web page open and write your bug description (then copy and submit inside your bug tracking tool).
Firebug Lite >>>
Firebug Lite is a tool for web developers, that allows you to edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
Click & Clean >>>
This extension allows you to delete your browser history, to clear cookies and cache but also to remove flash cookies.
Session saver-session manager >>>
With Session Manager you can manage multiple sessions, save and reload your browser state later.
Gmail checker >>>
Useful extension when you test forms or checkout pages when you need quick access to your email inbox.
Speed Tracer >>>
Speed Tracer will help you to identify and fix problems related to the performance of your web application.
Aviary Screenshot >>>
With this tool you can take a screen capture and edit it inside your browser. Great tool to use on Chrome if you are on MAC OS.
iMacros >>>
Use iMacros to automate some repetitious tasks while you test. Imacros can be combined with Speed Tracer, Firebug and other extensions tools. This extension will save you some time especially when testing e-commerce projects or signup/registration forms.