Not to beat a dead horse, but the Webkit guys released a DOM inspector in the nightlies. Here’s the announcement.

This thing traverses the DOM on the page you have loaded, which other can already do (I’m looking in your direction Firefox), but it has the nice addition/enhancement of also showing what styling is applied to the particular DOM object you have selected. Until now, you would have needed something like StyleMaster on OS X or CSSVista on Windows to easily handle element viewing. There’s bookmarklets that outlines block-level elements, but they’re not as fun or as nice looking.

Now if the webkit guys could just get on that one last Firefox-on-the-Mac-killer; a Javascript Debugger.