Basic one-level popups:
Multilevel popups (thanks to Porter Glendinning for the idea to use lists instead of horribly convoluted DIVs):
It's clean markup (check the markup!), it's a far sight easier to maintain than Javascript-driven menus, it's almost too easily reskinnable, the links can be given "tooltips" with a title attribute (try it with the links under "css/edge")... what's not to like? Besides the limited browser support, of course. Check this out:
All I did was change the styles-- well, in this case, I changed the ul elements to ol elements so that the styles would properly target the menu, but that's all!-- and got a new menu structure. How cool can it get?