<select name="test" onchange="showForm(this.value);">
<option value="">-----</option>
<option value="form1">Form 1</option>
<option value="form1">Form 2</option>
</select>
<div id="form1" style="display:none">
Form1
</div>
<div id="form2" style="display:none">
Form 2
</div>
<script type="text/javascript">
function showForm(id){
document.getElementById('form1').style.display='none';
document.getElementById('form2').style.display='none';
document.getElementById(id).style.display='block';
}
</script>