<script src="http://code.jquery.com/jquery-latest.js"></script>    
<script type="text/javascript">
	function addField() {
		$("#fields").append('<input type="text" name="text[]"><br />');
	}
</script>
<select name="name[]" onchange="addField()"><option value="1">a</option><option value="2">b</option></select>
<div id="fields">
	<input type="text" name="text[]"><br />
</div>