I recently came across a great toolbar for web designers. It has a whole bunch of cool features that will speed up testing and overall put more information at your fingertips when designing a website. I won't go through all the features, you will just have to check them out for yourself. Some of the features are:
The CSS features provide various tools related to the CSS on the page including displaying the styles applied to the selected element and live editing of the CSS.
The images features provide various image related tools including finding broken images and outlining images.
The information features provide various informational tools including displaying element information and viewing the document size.
The resize features provide the ability to resize the window to pre-defined or custom sizes and to display the current window size.
The tools features allow an unlimited number of configurable tools including validating the CSS and the HTML.
There view source features provide the ability to view the page source including viewing the generated source and viewing the source in an external application.
This toolbar is great and you should check it out if your developing for the web.