Hi

Since SilverStripe comes with jQuery, you could look for a dropline menu implementation for jQuery (like this one: http://www.dynamicdrive.com/style/csslibrary/item/jquery_drop_line_menu/). But actually you should be able to use any implementation you might find out there.

After including the required files, all that's left to do is to output the navigation in a way (in the template), that it will work with the dropline menu of your choosing.