Rescue Shortcuts for "Frozen" OS X?
From the article at http://support.apple.com/kb/ht3411

Press Command-Option-Esc to force quit, it brings up a dialog so you can choose which app to quit.

Command-Option-Shift-Esc for three seconds will force quit the front-most application.

Here is another helpful article: http://osxdaily.com/2012/03/02/force-quit-mac-apps/