[ZoNT]

<form>
<table>
<tr>
<td><input type="text" id="a" onkeyup="document.getElementById('result').innerHTML = (parseFloat(this.value)||0) + (parseFloat(document.getElementById('b').value)||0)"></td>
<td><input type="text" id="b" onkeyup="document.getElementById('result').innerHTML = (parseFloat(this.value)||0) + (parseFloat(document.getElementById('a').value)||0)"></td>
<td id="result"></td>
</tr>
</table>
</form>