//+ Jonas Raoni Soares Silva
//@ http://jsfromhell.com/classes/is-point-in-poly [v1.0]

function isPointInPoly(poly, pt){
	for(var c = false, i = -1, l = poly.length, j = l - 1; ++i < l; j = i)
		((poly[i].y <= pt.y && pt.y < poly[j].y) || (poly[j].y <= pt.y && pt.y < poly[i].y))
		&& (pt.x < (poly[j].x - poly[i].x) * (pt.y - poly[i].y) / (poly[j].y - poly[i].y) + poly[i].x)
		&& (c = !c);
	return c;
}

<script type="text/javascript">
//<![CDATA[

points = [
	{x: 0, y: 0},
	{x: 0, y: 50},
	{x: 50, y: 10},
	{x: -50, y: -10},
	{x: 0, y: -50},
	{x: 0, y: 0}
];

alert(isPointInPoly(points, {x: 10, y: 10}) ? "In" : "Out");

//]]>
</script>

//+ Jonas Raoni Soares Silva
//@ http://jsfromhell.com/geral/hittest [v1.0]

hitTest = function(o, l){
	function getOffset(o){
		for(var r = {l: o.offsetLeft, t: o.offsetTop, r: o.offsetWidth, b: o.offsetHeight};
			o = o.offsetParent; r.l += o.offsetLeft, r.t += o.offsetTop);
		return r.r += r.l, r.b += r.t, r;
	}
	for(var b, s, r = [], a = getOffset(o), j = isNaN(l.length), i = (j ? l = [l] : l).length; i;
		b = getOffset(l[--i]), (a.l == b.l || (a.l > b.l ? a.l <= b.r : b.l <= a.r))
		&& (a.t == b.t || (a.t > b.t ? a.t <= b.b : b.t <= a.b)) && (r[r.length] = l[i]));
	return j ? !!r.length : r;
};

<style type="text/css">
#menu{position: absolute; height: 500px; width: 200px; display: none; border: 1px solid #999; background: #eef;}
</style>

<div id="menu">
    <input type="button" onclick="hide();" value="Hide Menu" />
</div>
<form action="">
    <fieldset>
    <strong>DIV x SELECT</strong>
    <br /><select><option>DIV OVER SELECT DIV OVER SELECT DIV OVER SELECT</option></select>
    <input type="button" onclick="show();" value="Show Menu" />
    </fieldset>
</form>


<script type="text/javascript">
//<![CDATA[

var x = document.getElementById("menu");

//exemplo de como consertar o problema do select em cima do div, como só acontece no IE, é possível sniffar o browser...

function show(){
    x.style.display = "block";
    if(!x.ieFix)
        x.ieFix = hitTest(x, document.getElementsByTagName("select"));
    for(var i = x.ieFix.length; i; x.ieFix[--i].style.visibility = "hidden");

}

function hide(){
    x.style.display = "none";
    for(var i = x.ieFix.length; i; x.ieFix[--i].style.visibility = "visible");
}

//]]>
</script>