Firebug does this too. One thing Firebug does that I can't find in WebKit's developer tools: in Firebug, when you select an element and switch to the "Layout"

Firebug does this too. One thing Firebug does that I can't find in WebKit's developer tools: in Firebug, when you select an element and switch to the "Layout" tab, mousing over the various measurements will draw on-screen rulers and guides that show how the element lines up with other items on the page.