Select2

Select2 gives you a customizable select box with support for searching, tagging, remote data sets, infinite scrolling, and many other highly used options.

Select2 documentation
Stylesheet
<link href="assets/lib/select2/select2.min.css" rel="stylesheet">
Javascript
<script src="assets/lib/select2/select2.min.js"></script>
Basic Example
<div class="form-group">
  <label for="basic-example">Select an option</label>
  <select class="selectpicker" id="basic-example">
    <option>Afghanistan</option>
    <option>Albania</option>
    <option>Algeria</option>
    <option>American Samoa</option>
  </select>
</div>
Multiple Select
<div class="form-group">
  <label for="multiple-select">Select options</label>
  <select class="selectpicker" id="multiple-select" multiple data-options='{"placeholder":"Click to select your options"}'>
    <option>Afghanistan</option>
    <option>Albania</option>
    <option>Algeria</option>
    <option>American Samoa</option>
  </select>
</div>