(defun rcy-browse-url-default-macosx-browser (url &optional new-window)
(interactive (browse-url-interactive-arg "URL: "))
(let ((url
(if (aref (url-generic-parse-url url) 0)
url
(concat "http://" url))))
(start-process (concat "open " url) nil "open" url)))
(setq browse-url-browser-function 'rcy-browse-url-default-macosx-browser)
You could of course also use builtin function M-x ffap or bind it to some keys.