Web Inspector, a DOM inspector for WebKit
Very cool feature from the WebKit team, coming soon to a Safari near you – the Web Inspector:
The Web Inspector highlights the node on the page as it is selected in the hierarchy. You can also search for nodes by node name, id and CSS class name.
One of the unique features of the inspector is the ability to root the DOM hierarchy by double clicking a node to dig deeper. This lets you easily manage large nested pages and only focus on a particular sub-tree with minimal indentation.