Gray out

<form>

Where do you live?
<select id="choice" size="0" onchange="check();">
<option value="A">A</option>
<option value="B">B</option>
<option value="C">C</option>
<option value="Other">Other</option>
</select>
<input type="text" id="other_name" disabled="disabled" />

</form>

<script type="text/javascript">
function check() {
if (document.getElementById('choice').value == 'Other')
    document.getElementById('other_name').disabled = false;
else
    document.getElementById('other_name').disabled = true;
}
</script>