The outspoken Roger Johansson reminds us that there are alternative web debuggers besides the popular Firebug for Firefox:
XRAY lets you inspect the properties of any element on the page by opening a floating palette that displays the element type, id, class, position, size, border, margin, and padding of any element you click on. It also displays the element’s inheritance hierarchy.
MRI opens a similar floating window and has an input field where you can enter a CSS selector. When you click the MRI button, MRI highlights the elements on the page that are matched by the selector. MRI also works the other way around. Click anywhere on the page and it will suggest CSS selectors for the element you clicked on.
Safari 3 also ships with their own version of these tools called Web Inspector. Additionally Firebug provides a lite version that runs in Internet Explorer, Safari, and Opera. One thing thats not clear from the Firebug Lite page is whether or not it provides the same information about AJAX requests that the Firefox extension does. (A critical feature in my mind.)