Based on simple, semantically correct XHTML Lists.
Multiple levels. As many as you like.
Readily customisable via CSS to fit into your graphic design.
Valid XHTML.
Valid CSS.
Doesn't require a frameset.
Put it directly in your pages (or via embedding techniques like XSLT transforms;
Server Side includes; or when all browsers are up for it -> <object>)
Easy to use Javascript. You only need 3 lines of code in your XHTML
source code to initialize the menu (by calling Javascript in an external file).
Optionally specify which parent nodes are to be expanded upon load or user executed
"Default Expand" command. Do this by simply adding a class name in your list.
Tested in most popular browsers (at Aug 2005). IE 6.0; Firefox 1.0