We named this script "HV menu" for its ability to be laid out both horizontally (as a top menu) and vertically (as a side menu), but it might as well be for its highly versatile nature. The menu's impressive list of features makes it one of the best scripts of its kind today:

Ability to act either as a horizontal or vertical menu (through toggle of a single variable)
Multi-level submenus supported (ie: 3 levels down)
Menu interface completely customizable (color, alignment etc)
Drop down in another frame supported (frames support)
Functional in all major DHTML browsers- IE4+, NS4, NS6+, Opera7+
New features in version 5.5 (compared to v5.0):

Menu status clarified- HV menu found on Dynamic Drive may be used on both commerical and non commerical sites. This applies EXCLUSIVELY to script found on Dynamic Drive, and not author's site.
Menu can be vertically static
Menu can be horizontal justified
Selected path can stay highlighted
More control over arrows.
Fixed relative positioning bug in Mozilla

SoftDrawer jsTree is a javascript tree menu. It offers not only a lot of interesting and customizable features, like cool animation effects (try to collapse/expand nodes), but also full cross-browser compatibility and the easiest way to setup and configure your javascript menu. Besides this menu, we are glad to offer a Professional version of SoftDrawer jsTree with a lot of new cool features like relative positioning, animation, individual item or level formatting, and more, more, more.

Unlimited number of menus on a single page all independently configurable
Unlimited menu depth, each level and item can be independently configured
Supports frames with border overlapping. Any frameset layout will do (UPDATED)
Can be used as a context menu - just set root level to be hidden initially (NEW)
Configurable geometry makes it possible for items to drop down the way you like it - vertical, horizontal menu, overlapping etc.
Lacking menu level scope setting values are inherited from a parent level
Each menu block and menu item may be positioned and sized independently
Pure relative positioning. Put menu anywhere in the HTML code and see menu at exactly the same place on the page
Optional absolute positioning - menu can be drawn out relative to left upper page corner (NEW)
All styles are configurable: fonts, colors, distances, decorations etc.
Layout settings (vertical menu or horizontal menu) available for each menu level
Submenus can be expanded on click (NEW)
Optional smart submenus relocation. Menu moves blocks to make them visible when they go out of low resolution screen
Base z-index - menu can be easily tuned up to overlap any other document layers (NEW)
Any rollover or onclick effect can be implemented
Custom event hanlers for each menu item can be defined (NEW)
Any item can contain any html (tables, pictures etc.) making possible to embody most amazing design ideas
Link targets (other frame, new window, parent frame) and tool tips are supported
Appropriate window status bar message can be assigned to be displayed once a menu item is mouseovered
Customizable attractive transparency and transition effects (IE 5+) (UPDATED)
All popular browsers supported (compatibility table)
Whole menu item area is clickable (not text caption only as in most other javascript menus)
Size optimized to reduce network load
Minimum efforts to migrate to if you already have Tigra Menu
Tons of support materials and friendly competent person on the other side ready to help when you have a question
PHP and ASP samples for dynamic menu generation from the database or file system
Reasonable price (only $49)

XML Menu is a state-of-the-art, client-side web menu builder that uses XML to describe menu content and CSS to describe menu presentation (look and feel). XML Menu is powered by AJAX.

The javascript code includes horizontal and verticle popup menus,
a cascading menu, two tabular menus and a mouseover menu. All javascripts
work in IE, Mozilla, Firefox, Opera, Sefari and Netscape browsers.