sawfishstuff.el

(defun get-window-by-class (class)
  "Return a window with the given class."
  (first (filter-windows #'(lambda (w) (equal (window-class w) class)))))

(defun gimme (cmdline class)
  "If a window matching CLASS is found switch to it, else launch cmdline."
  (if class
      (let ((wnd (get-window-by-class class)))
	(if wnd
	    (display-window wnd)
	  (system cmdline)))
    (system cmdline)))

(defun gimme-emacs ()
  (gimme "emacs &" "Emacs"))

(bind-keys global-keymap "W-e" gimme-emacs)