Directory: Menu

JavaScript User Interfaces is a reusable module-based JavaScript library for making graphical OS-like Dynamic HTML , or DHTML , objects such as windows, popup menus, taskbars, etc.

DOM Tooltip allows developers to add customized tooltips to Web pages. The tooltips are controlled through style class definitions and respond to events such as "mouseover", and avoids possible collisions with form elements such as select boxes and screen edges. While originally designed to create context tooltips, it is also possible to create a wide variety of dynamic layers, such as embedded windows, context menus, and hidden blocks. Additional features include sticky tips, tooltip fading, lifetime, relative positioning, class assignments, width adjustments, mouse dragging, captions, directionality, offset adjustments, adjustable activate/deactivate delay times, snapping to grid, fate adjustment (hide or destroy), and references to created tips. It supports Mozilla/Netscape6+, IE 5.0+, Konqueror, and Opera 7.

RelativeLayers is a Javascript library for creating dynamic designs that adapt themselves to the user's browser dimension. It supports Netscape, Internet Explorer, Mozilla, Konqueror and Opera. Lots of website design concepts have been re-thought and optimized.

PHP Layers Menu is a hierarchical dynamic menu system that allows to prepare "on the fly" dynamic HTML menus relying on the PHP scripting engine for the processing of data items.

NanoTree is a JavaScript tree, published under the LGPL License, which is developed to work in (at least) Internet Explorer and Mozilla
If other browsers will be added later, this will only be a plus.
The code is flexible, and it's easy to add to- and alter it. Also the code was made so it's easy to auto-generate with serversidescripts like PHP/JSP/ASP.

Xmenu-xlayer is a versatile, javascript driven DHTML menu that works on
Internet Explorer Windows 5 and later (5, 5.5, 6)
Gecko (Mozilla, Mozilla-Firebird, Galeon, Netscape 6, 7, etc.)
Opera 6, 7
Internet Explorer Macintosh 5 and later (5, 5.1, 5.2)
Safari
Konqueror 3.x
Netscape 4.x

It comes in 3 basic modes:
horizontal
vertical
collapsing