dired-ediff

; ----------------------------------------
;; dired ediff
; ----------------------------------------
(defun my-dired-ediff-marked-files ()
  "Run ediff on 2 or 3 marked dired files."
  (interactive)
  (let ((marked-files (dired-get-marked-files)))
    (if (= (safe-length marked-files) 2)
        (ediff-files (nth 0 marked-files) (nth 1 marked-files))
      (if (= (safe-length marked-files) 3)
          (ediff3 (nth 0 marked-files)
                  (nth 1 marked-files)
                  (nth 2 marked-files))
        (error "mark either 2 or 3 files.")))))

(define-key dired-mode-map "=" 'my-dired-ediff-marked-files)