codice:
<script>
function loadPage(list) { 
location.href = list.options[list.selectedIndex].value
}
</script>

<form>
<select name="select" onChange="loadPage(this.form.elements[0])">
<option value="NIL">...</option>
<option value="a.htm">A</option>
<option value="b.htm">B</option>
<option value="c.htm">C</option>
</form>