http://jsfiddle.net/tAjNz/
<input type="text" />
<select id="autoPop" multiselect="true"></select>
<script>
// define a data source (ajax or on page load);
myJson = [{value:1,text:'Item One'},{value:1,text:'Item Two'},{value:1,text:'Item Three'},{value:1,text:'Item Four'}];
$('input[type="text"]').keyup(function(){
//Optionally update the data source when a user starts typing or use the predefined source.
//Populate the select list
$sel = $('select');
$sel.html('');
var $this = $(this);
$.each(myJson,function(k,v){
if(v.text.toLowerCase().indexOf($this.val().toLowerCase()) > -1) {
$sel.append('<option value="' + v.value+ '">'+ v.text+'</option>');
}
});
});
</script>