Как вариант...
<!DOCTYPE html>
<html>
<head>
<script src="http://code.jquery.com/jquery-latest.js"></script>
<!--
<link rel="stylesheet" type="text/css" href="tmp.css" />
-->
<style type="text/css">
</style>
<script type="text/javascript">
$(document).ready(function (){
$('#vzialtext').click(function (){
var l,val;
l=$('#container :text').length;
val='<input value="seria ' + l +'" size="20" />';
$('#container').append(val);
});
});
</script>
</head>
<body>
<div id='container'>
<input type="text" id="text" size="20" value="1s" />
</div>
<input type="button" id="vzialtext" value="+" />
</body>
</html>