<select class="dropdown" name="list" onchange="window.location=this.value;">

    <option value="http://www.taobao.com">淘宝网</option>

    <option value="http://www.tmall.com" selected=selected>天猫</option>

    <option value="http://ju.taobao.com">聚划算</option>

    <option value="http://www.fliggy.com">飞猪</option>

 </select>