Safari / Webkit Developer Tools
Safari ships with awesome developer tools (similar to Firebug), but it’s disabled by default. To activate the developer tools in Mac OS X, quit any running instances of Safari (or Webkit), open up the Terminal and type
defaults write com.apple.Safari WebKitDeveloperExtras -bool true
defaults write com.apple.Safari IncludeDebugMenu 1
defaults write com.apple.Safari IncludeDebugMenu 1