As described in this previous hint, you can navigate a site's structure in Safari by command-clicking the title bar.
I accidentally discovered that you can get the same behavior by clicking the title bar with the middle button/scroll wheel of a two-button scroll wheel mouse. While many other apps (Finder, Preview, TextEdit, etc.) use the title bar command-click, the scroll-wheel-click method seems to be unique to Safari.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060330121002976