Sometimes, you may want to add a cool drop down menu that will show all existing categories in your WordPress site. Here is a simple way of doing it. Copy and paste the code below into the index.php or sidebar.php file.

wp_page_categories_dropdown.php

<form action="<?php bloginfo('url'); ?>/" method="get">
 <?php
 $select = wp_dropdown_categories('show_option_none=Select category&show_count=1&orderby=name&echo=0');
 $select = preg_replace("#<select([^>]*)>#", "<select$1 onchange='return this.form.submit()'>", $select);
 echo $select;
 ?>
 <noscript><input type="submit" value="View" /></noscript>
 </form>