There are several GUI programs allowing different "tricks" on OS X but one of the tricks - enabling Debug menu in Safari, is too easy to require an extra mile of installing a GUI application.

Debug menu in Safari delivers a wealth of additional information, essential for web developers, web designers and other enthusiasts. Using it you can examine DOM, get web page metrics, change user-agent etc.

To enable Debug menu, open Terminal window and type: \

 $ defaults write IncludeDebugMenu 1

\ Restart Safari \ enjoy :)