jQuery multiselect drop down menu

Answer 1

Are you looking to do something like this http://jsfiddle.net/robert/xhHkG/

$('#transactionType').attr({
    'multiple': true,
    'size' : 10
});

Put that in a $(function() {...}) or some other onload

Edit

Reread your question, you're not really looking for a multiple select... but a dropdown box that allows you to select multiple. Yeah, probably best to use a plugin for that or write it from the ground up, it's not a "quick answer" type deal though.