<script>
function crear() {
var tbx = document.createElement('input');
tbx.type = 'text';
tbx.name='a';
document.form1.appendChild(tbx);
}
</script>
<form id='form1' name='form1' >
<input type='button' onClick="crear()" value="Crear Text"><br /><br />
<input type='text' value="">
</form>