I am a bit surprised that no one has mentioned the freeware application Safari Enhancer. For all of those who wish to fiddle with Safari's hidden features without editing files themselves, this is the application.
It can reveal the debug menu, set the minimum font size, underline links and import bookbarks from other browsers.
[Editor's note: The features in Safari Enhancer can be enabled through the command line, but Enhancer certainly makes it easier!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030113060911226