This is simple example to get nested category list. We can easily create a left navigation and a drop-down menu with the Retrieved HTML. Follow the code :

<?php

set_time_limit(0);
error_reporting(E_ALL ^ E_NOTICE);
ini_set(“display_errors”,’On’);

$rootDir = “”; // Root Directory Path
include($rootDir.”app/Mage.php”);
Mage::app(“default”);

$rootcatId= Mage::app()->getStore()->getRootCategoryId(); // get default store root category id
$categories = Mage::getModel(‘catalog/category’)->getCategories($rootcatId); // else use default category id =2

function  show_categories_tree($categories) {
    $array= ‘<ul>’;
    foreach($categories as $category) {
        $cat = Mage::getModel(‘catalog/category’)->load($category->getId());
        $count = $cat->getProductCount();
        $array .= ‘<li>’.
        ‘<a href=”‘ . Mage::getUrl($cat->getUrlPath()). ‘”>’ .
                  $category->getName() . “(“.$count.”)</a>n”;
        if($category->hasChildren()) {
            $children = Mage::getModel(‘catalog/category’)->getCategories($category->getId());
             $array .=  show_categories_tree($children);
            }
         $array .= ‘</li>’;
    }
    return  $array . ‘</ul>’;
}
echo  show_categories_tree($categories);

?>