Ti rispondo qui anche se si tratta di js...
In jquery lo farei così:
codice:
<html>
<head>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<script>
$(function(){
$('#my_select').bind('change', function () {
var url = $(this).val();
window.location = url;
return false;
});
});
</script>
</head>
<body>
<form action=ìì>
<select name="siti" id="my_select">
<optgroup label="siti per internet">
<option value="http://www.html.it">www.html.it </option>
<option value="http://freephp.html.it">frephp.html.it </option>
<option value="http://freasp.html.it">freasp.html.it </option>
</optgroup>
<optgroup label="risorse per divertimento">
<option value="http://..........">videogiochi </option>
<option value="http://............">giochi on line </option>
</optgroup>
</select>
</form>
</body>
</html>