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:

  1. eric
    1. Speaking
    2. Writing
    3. Books
      1. CSS:TDG
      2. Eric Meyer on CSS
        1. About the book
        2. Advance praise
        3. Order a copy!
        4. Projects
        5. About the author
      3. CSS2 ProgRef
      4. CSS Pocket
    4. CSS info
    5. css/edge
      1. complexspiral
        1. ...devolved
        2. ...distorted
      2. Pure CSS popups
      3. curvelicious
      4. slantastic
        1. slanty claus?
      5. ragged float
      6. boxpunch
  2. other
    1. Across the Middle Kingdom
    2. Our Trip to Cancun
    3. Our House
    4. The Funny Bits

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?