Example

Starting with this multiple select element with 6 total values, 2 are selected.

<select id="whopper" multiple="multiple" size="4">
  <option value="mayo">Mayonnaise</option>
  <option value="lett" selected="selected">Lettuce</option>
  <option value="tomo">Tomatoes</option>
  <option value="onin" selected="selected">Onions</option>
  <option value="pckl">Pickles</option>
  <option value="kchp">Ketchup</option>
</select>

Which normally looks like this:

Adding this unobtrusive Javascript to the header <script>:

jQuery(document).ready(function() {
  $('#whopper').multi_single_select({list_class:'toppings', title:'Topping'});
});

converts the above element to this (it's a list element with some styles):

If the select has a label for it, the header string 'Options...' will be replaced by it