Mod 'hideshow'

(defun ruby-custom-setup ()
; [other stuff omitted...]
   (add-to-list 'hs-special-modes-alist
                '(ruby-mode
                   "\\(def\\|do\\)"
                   "end"
                   "#"
                   (lambda (arg) (ruby-end-of-block))
                   nil
                   ))
   (hs-minor-mode t)
)

(add-hook 'ruby-mode-hook 'ruby-custom-setup)