nav despleglable
<script type="text/javascript">
function show(element) {
if(element.value=='Other')
document.getElementById('other').style.display='block';
else document.getElementById('other').style.display='none';
}
</script>
<form action="">
<select name="some_name" onchange="javascript:show(this)">
<option value="">--Please Choose--</option>
<option value="Other">Other</option>
</select>
<div id="other" style="display:none">
<input type="text" name="other_name">
</div>
</form>