For my money, Safari's Web Inspector has been a strong rival to the excellent Firebug that Firefox deveopers use. Its capabilities and usability have been growing by leaps and bounds since it was first introduced in January 2006.

I wrote a brief article on the enhancements that came in August 2006, and since then the improvements have just kept coming:

1. June 2007, total redesign with beaucoup added features.
2. December 2007, adding inline CSS editing, a database browser, downloadable fonts, several CSS properties, and more.
3. September 2008, redesigned interface and way too many terrific features to list here. Go read the WebKit blog article for details.

At this point, I don't think there's any question among those of us who've used both Firebug and Web Inspector that the bar has now been raised far beyond what's available to Firefox developers.

Note: At this time, the new web inspector--like the hugely faster Squirrelfish JavaScript engine--is only available in the nightly WebKit builds. But that doesn't mean you can't use it in place of Safari!