Birdman there are many ways, but some of us don't want to tell all (I'm proud of one particular menu I built using CSS ;))...
however a good starting point is ALA's Taming Lists [alistapart.com]
and from then even without using lists just using:
and giving it padding, width, borders then giving it different a:hover properties...it's up to you!
I was adamant I wanted a .gif to appear on hover and even though Opera doesn't support it, I got around it, but then even today I found a superb menu that looks like a 3D effect and is done purely using colors and borders so look around find a menu you like (without dropdowns as unfortunately this is not possible yet! - further menu choices that is, descriptions of links are possible) and try to reproduce.
Sticky a link if you have a particular idea in mind or post link....
<edit>re CSS pop ups [meyerweb.com] are possible to a certain degree</edit>
[edited by: SuzyUK at 12:28 am (utc) on Jan. 16, 2003]