codice:
<script type="text/javascript"> 
<!--
function sel_all(obj) { 
  for (var k=0;k<obj.options.length;k++) {
    obj.options[k].selected=true;
  } 
} 
//-->
</script> 
...  
<form>
<select name="sel" multiple="multiple"> 
<option>a</option> 
<option>b</option>
<option>c</option>
<option>d</option>
</select> 
<input type="button" value="Seleziona tutti" onclick="sel_all(this.form.sel)" /> 
</form>
ciao