<kbd>Command ⌘ </kbd> + <kbd>Alt</kbd> + <kbd>Esc</kbd> opens the "Force Quit Application" 
dialog, which lets you select and kill running applications. 

If that does not work, 

<kbd>Command ⌘</kbd> + <kbd>Alt</kbd> + <kbd>Shift</kbd> + <kbd>Esc</kbd> held down for 5 seconds kills the foremost application.