Safari has included the webkit tools that the Webkit Open Source Organization has been working on. It looks very nice, but still not quite as powerful as the web tools available to Firefox. You can't actually edit the css on the page through the tool, but you can get a nice look at what styles are being appled to what elements. You right click on elements and say inspect element. The Web Inspector opens to that element. It has four tabs - Node, Style, Metrics and Properties. Using the tabs you can see how the browser is rendering the elements.

The Web Inspector floats above your document and is semi-transparent with a black background.