The X-Ray extension let’s you see the tags on a page without viewing the sourcecode.
One installed the X-Ray command is available by right-clicking as well as in the Tools menu. When applied to a page it will display the documents tags, including class and ID names. This can help you see how a website was constructed without having to go back and forth between the sourcecode and the page in your browser.
This is the companion extension to Professor X, although either can be installed and used on their own. They were created to help people learning web design at various levels of experience.
Update – August 14, 2008