I am always fascinated with themes using drop down menu and even better if it support more than 1 level of submenu. I would actually say 2-level submenu would be very much sufficient for most usage. Only one of my blog currently uses CSS Submenu because it has a lot of categories and the CSS Menu helps visitor to navigate around easily.
So to create a CSS Menu may just tough especially if you want to incorporate round corner, picture, mouse over and other graphical effect. I have found this great software which is free to use. It does not use java script and 100% css driven menu. The best is that if provides a GUI for you to configure your menu.