Mac: If you're confronted with the spinning beach ball of death and the application you're using isn't responding, just press Cmd+Opt+Shift+Escape to quit the application in front. You can also press Cmd+Opt+Esc to bring up the Force Quit menu and select the app you want to kill.
You can also force quit applications by clicking and holding on their dock icon (or right-click) until a menu appears with Force Quit as the bottom option. Think of Force Quit as "End Task" for Mac users.
Shortcut of the Day highlights a great shortcut on various platforms from the desktop to mobile devices to the web. You can find more shortcuts here.