I regularly switch my Safari user agent to "iPad" so I can play embedded YouTube videos without errors. It seems that many embedded videos still require flash if they see I am using Safari. Switching to the iPad user agent causes the videos to be played as HTML5 without need of the flash suppository.
To aid in this switching, I developed a couple of Keyboard Maestro macros to allow me to swap at will with just the hot key combo ctrl-opt-U.
The macros require the "Develop" menu item to be enabled. The setting is located in the Safari preference at the bottom:
Here's the basic macro for the iPad user agent:
Here's a bit more complicated version for using the Chrome user agent. In this case KM selects the "Other..." menu and then pastes in the Chrome user agent. For a complete list of user agents, this is a terrific source.
The macros are restricted to Safari only. Since all the macros are triggered by the same hotkey combo, I just hit ctrl-opt-U and then the number of the agent I want.