http://github.com/michael/multiple_select/
<script type="text/javascript" src="javascripts/mootools.v1.11.js"/>
<script type="text/javascript" src="javascripts/multiple_select.js"/>
<select name="countries" class="multiple_select" multiple="multiple">
<option value="1" selected="selected">Austria</option>
<option value="2">Czech</option>
<option value="3">France</option>
<option value="4">Germany</option>
<option value="7">Italy</option>
<option value="5">Spain</option>
<option value="6">Switzerland</option>
</select>
<script language="javascript">
$$('.multiple_select').each(function(el){
new MultipleSelect(el);
});
</script>