try this :
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<script type="text/javascript"><!--
function getVal(e) {
var targ;
if (!e) var e = window.event;
if (e.target) targ = e.target;
else if (e.srcElement) targ = e.srcElement;
if (targ.nodeType == 3) // defeat Safari bug
targ = targ.parentNode;
alert(targ.innerHTML);
}
onload = function() {
var t = document.getElementById("main").getElementsByTagName("td");
for ( var i = 0; i < t.length; i++ )
t[i].onclick = getVal;
}
</script>
<body>
<table id="main"><tr>
<td>1</td>
<td>2</td>
<td>3</td>
<td>4</td>
</tr><tr>
<td>5</td>
<td>6</td>
<td>7</td>
<td>8</td>
</tr><tr>
<td>9</td>
<td>10</td>
<td>11</td>
<td>12</td>
</tr></table>
</body>
</html>