Here’s a neat trick, for those of you using Safari. If you want to see the debug menu (which includes goodies like a DOM tree viewer and a “snippet viewer”, which lets you take a snippet of HTML and view it as Safari will render it, sort of like a split-screen WYSIWYG editor) just shut down Safari, open up Terminal and type:
defaults write com.apple.Safari IncludeDebugMenu 1
And start Safari again. To turn it off again, run the same command with a 0 instead of a 1.