[bes]

<table> 
	<tr> 
		<td class="sumItg">1</td>
		<td class="test">2</td>
		<td class="sumItg">2</td> 
		<td class="test">3</td>
		<td class="sumItg">3</td> 
		<td class="test">4</td>
	</tr> 
</table> 
<span id="parent"></span>
<script> 
window.onload = function () {
	var sumItg = 0;
	var td = document.getElementsByTagName('td');
	var i = td.length; 
	while(i--) {
		if(td[i].className == 'sumItg') { 
			sumItg += td[i].innerHTML * td[i].nextElementSibling.innerHTML;
		}
	}
	document.getElementById('parent').innerHTML = sumItg;
}
</script>

не забудь обеспечить кроссбраузерность nextElementSibling