Nick Rigby's article on using lists, CSS and a little JS to create fly-out hierarchical menus.

Anyone who has created drop-down menus will be familiar with the large quantities of scripting such menus typically require. But, using structured HTML and simple CSS, it is possible to create visually appealing drop-downs that are easy to edit and update, and that work across a multitude of browsers, including Internet Explorer. Better still, for code-wary designers, no JavaScript is required! (Actually, a tiny bit of JavaScript is needed, but it’s not what you think.)

Drop-Down Menus, Horizontal Style