Command - Option - Escape is working for me...
Or,
Click on the Apple logo [ top left ] and choose Force Quit
Additionally, Command, Option, Shift, Esc will force quit an app when the window or full screen is active.
...which I only discovered recently. One trick though, you must hold the keys down for several seconds before it will work.
And of course all of these force quit approaches apply to OS X before Mavericks as well!