The YUI toolkit includes an entire family of Menu widgets with support for:

• Screen Reader Accessibility
• Keyboard and Mouse Navigation
• A Rich Event Model that Provides Access to all features
• Support for Progressive Enhancement; Menus can be created from simple html  structures or 100% in javascript.

More Information:

http://developer.yahoo.com/yui/menu/ 

• YUI Color Picker
• Building a Dynamic Form with YUI
• Build a YUI Panel from Content
• Building a Form in a Modal Panel with YUI
• 5 Open Source Rich Text Editors
• Getting and setting values for jQuery widgets
• YUI Calendar Popup from Text Input
• Taking The Web To The Desktop Part 3 Widgets
• YUI 2.4 has landed
• Determining Position with YUI