e' dhtml ed e' lo stesso effetto che usa poste.it
che x fini didattici riporto qui sotto ridotto all' osso
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>menu' poste.it</title>
<style type="text/css">
#object1{position:absolute;left:0px;top:83px;width:98px;height:345px;z-index:1}
</style>
<script language="JavaScript" type="text/javascript">
<!--
function setVariables(){
	if (navigator.appName == "Netscape") {
		if (parseInt(navigator.appVersion) >= 5){
			v=".top=";
			h=".left=";
			dS="document.getElementById(\"";
			sD="\").style";
			y="window.pageYOffset";
			x="window.pageXOffset";
		}
		else {
			v=".top=";
			h=".left=";
			dS="document.";
			sD="";
			y="window.pageYOffset";
			x="window.pageXOffset";
		}
	}
	else {
		h=".pixelLeft=";
		v=".pixelTop=";
		dS="";
		sD=".style";
		y="document.body.scrollTop";
		x="document.body.scrollLeft";
	}
	checkLocationA()
}

movex=0,movey=0,xdiff=0,ydiff=0,ystart=0,xstart=0

function checkLocation(){
	object="object1";
	yy=eval(y);
	xx=eval(x);
	ydiff=ystart-yy;
	xdiff=xstart-xx;
	if ((ydiff<(-1))||(ydiff>(1))) movey=Math.round(ydiff/10),ystart-=movey
	if ((xdiff<(-1))||(xdiff>(1))) movex=Math.round(xdiff/10),xstart-=movex
	if (ystart>=83){
		eval(dS+object+sD+v+(ystart+10));
	} else {
		eval(dS+object+sD+v+83);
	}

	eval(dS+object+sD+h+xstart);
	setTimeout("checkLocation()",10);
}

function checkLocationA(){
	ystart=eval(y);
	xstart=eval(x);
}

//-->
</script>
</head>
<body onLoad="setVariables();checkLocation()">
<div id="object1">
il tuo menu' qui
</div> 
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</body>
</html>
ciao