The CSS and JavaScript:

<style type="text/css">

#cssdropdown, #cssdropdown ul {
padding: 0;
margin: 0;
list-style: none;
}

#cssdropdown li {
float: left;
position: relative;
}

.mainitems{
border: 1px solid black;
background-color: #FFEEC6;
}

.mainitems a{
margin-left: 6px;
margin-right: 8px;
text-decoration: none;
}

.subuls{
display: none;
width: 10em;
position: absolute;
top: 1.2em;
left: 0;
background-color: lightyellow;
border: 1px solid black;
}

.subuls li{
width: 100%;
}

.subuls li a{
text-decoration: underline;
}

#cssdropdown li>ul { /* to override top and left in browsers other than IE, which will position to the top right of the containing li, rather than bottom left */
top: auto;
left: auto;
}

#cssdropdown li:hover ul, li.over ul { /* lists nested under hovered list items */
display: block;
}

#restofcontent { /*wrap rest of content of the page inside this div*/
clear: left;
}

</style>

<script type="text/javascript"><!--//--><![CDATA[//><!--
startList = function() {
if (document.all&&document.getElementById) {
cssdropdownRoot = document.getElementById("cssdropdown");
for (x=0; x<cssdropdownRoot.childNodes.length; x++) {
node = cssdropdownRoot.childNodes[x];
if (node.nodeName=="LI") {
node.onmouseover=function() {
this.className+=" over";
}
node.onmouseout=function() {
this.className=this.className.replace(" over", "");
}
}
}
}
}

if (window.attachEvent)
window.attachEvent("onload", startList)
else
window.onload=startList;

//--><!]]></script>

The HTML:

<ul id="cssdropdown">

<li class="mainitems">
<a href="http://www.cssdrive.com">CSS Gallery</a>
<ul class="subuls">
<li><a href="">Gordon Mac</a></li>
<li><a href="">Juiced Thoughts</a></li>
<li><a href="">The Daily Flight</a></li>
<li><a href="">GrapeFruit</a></li>
</ul>
</li>

<li class="mainitems">
<a href="http://www.cssdrive.com/index.php/examples/">CSS Examples</a>
<ul class="subuls" style="width: 15em">
<li><a href="">CSS2 Generated Content</a></li>
<li><a href="">Custom scrollbar colors</a></li>
<li><a href="">Markerless and "no indent" Lists</a></li>
</ul>
</li>

</ul>

<div id="restofcontent">
<br />
"
</div>