Enable Debug Menu in Safari
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 com.apple.Safari IncludeDebugMenu 1
\ Restart Safari \ enjoy :)