Thanks, that is ****ing nifty, I never used it to look for this.
Well allow me to blow your mind a little bit more
Not only can you right click on an element and "Inspect Element". Once you have the pane open, you can hover your mouse over DOM elements and Safari will highlight the hovered segment in blue and even give size calculations.
Clicking on an element in the DOM will show the hierarchy of CSS rules that are applied to that element and which CSS files they came from. You can live toggle on and off rules, as well as live edit them. The selector rules will automatically apply to any element they should apply, so its easy to test styling on things like lists or paragraphs. You can also edit the just the one elements style, which is great for just testing things like putting a margin or border around a select element.
Going over into the Console tab (or if the bar isn't up hitting command+option+c) here you can type, or paste in javascript and have it automatically run on the page. This is great for hand tuning some simple javascript like finding a form and copying its value or something. If the Javascript has return types, it can be very useful, like typing "document.forms" will display every form tag with little drop down triangles.
You'll note that you can use any JS library loaded by the current document. Which is great, because usually jQuery is available to you. And if it isn't you can just create and append a new script element to load it from google.