As evidenced by the comments below this is apparently a third party product.  What mystifies me is how it came to be on my machine, since I did not download and install it.  Who knows.  If you are a Safari user who would like this addition hop over to here and download it.

I updated my home machine to 10.4.2 tonight and after launching Safari I noticed something odd:

Eh? Web Dev additions?  What in the world is that?

So being who I am, I started snooping around looking for what these "additions" actually did.  I soon found out, and much coolness ensued.  So lets take a look at what is new:

The New and Improved Edit Menu^TM

Pop open the Edit menu and you will find a new section falling between Autofill and Find:

The first two are pretty straightforward, a javascript console that is less functional than the one in Firefox but still a step forward, and Show CSS Styles, which I have not been able to get to work yet... still looking into it.

The really exiting stuff is what comes next, Disable and Outline.  Let's take a closer look:

The Disable Menu or out, out damn images!

Yeah that is finally some Development Hotness for Safari users, yes Firefox has had if for ever... yada yada.  The point is that Safari is finally coming to the table to play with the big boys.  So here is a shot of this site with images turned off:

Very satisfying to see your layout not change when images are turned off.  Mmmmm.. CSS good.  So moving on we have:

The Outline Menu or Keep those crayons inside the lines!

I have always found this the most useful bit of the web-developer extension in Firefox.  And of course my favorite is Block Level Elements.  For a look at how useful it can be, take this shot of the divitis that Sillyness suffers from... just embarrassing I need to do something about this:

Just sad really.  So anywhoo, this is some serious love that is being bestowed upon the unwashed masses that are still trying to use Safari as our main development browser.  Much love Hyatt and team for a jorb well done.

And for the love of Mike get out there and play around with it a bit, see what you find.  Might actually learn a thing or three... I know I have already.