Another alternative that, AFAICS, doesn't use anything external to emacs

    ;;; Full-screen mode
   (defun toggle-max-window ()
      (interactive)
      (set-frame-parameter nil 'fullscreen
                       (if (frame-parameter nil 'fullscreen)
                           nil
                         'fullboth)))
   (global-set-key (kbd "<C-M-return>") 'toggle-max-window)