<select id="filter" name="filter">
    <option value="-1">Filter items</option>
    <option value="sc">South Carolina</option>
    <option value="nc">North Carolina</option>
</select>



<div id="container">

<div class="box state-nc"></div>
<div class="box state-sc"></div>
<div class="box state-nc"></div>
<div class="box state-sc"></div>
<div class="box state-nc"></div>
<div class="box state-sc"></div>
<div class="box state-nc"></div>
<div class="box state-sc"></div>
</div>

var $all_items = $('.item');
var state = $dropdown.val();
var $items = $('.item[class!="state-' + state + '"]');
// or this:
// var $items = $('.item[data-state!="' + state + '"]');
if ($items.size() == $all_items) {
  $all_items.fadeOut();
  // show "no options found" instead
} else {
  $all_items.show();
  $items.fadeOut();
}