Here are the top 5:
Web Developer Toolbar: This one is a collection of all the necessary and useful actions that we are required to carry out to test the skillfulness and state of the web application that is being developed.
Measureit: Frequently while the development of a web application is going on it becomes essential to keep track of the dimensions of DIV or for that matter other HTML elements in case you are designing the page as per the requirement for the purpose of eliminating any glitches as they happen.
Colorzilla: This one can be useful for the developer to pick the color he wants from any part of the browser and swiftly adjust this color and paste it on a different program like GIMP or Photoshop and can be used to design visuals for the website. You can also make use of the feature for zooming a page on the page and measuring the distance that two points have between them.
IE-tab Plus: This one is a great aid when you are in the process of designing a web application, It helps developers take care of the minutest aspect on the application.