Quoting from the Yahoo YUI website:
"The YUI Library is a set of utilities and controls, written in JavaScript, for building richly interactive web applications using techniques such as DOM scripting, DHTML and AJAX. YUI is available under a BSD license and is free for all uses. The YUI project includes the YUI Library and two build-time tools: YUI Compressor (minification) and YUI Doc (documentation engine for JavaScript code)."
On this page is a drop down navigation menu built with Yahoo YUI library components. All of the components are customizable, of course. The one on this page is presented as is, but my next example is modified considerably from the original.