This small chunk of code recursively outputs all (active) categories within a Magento store into a nested, clean, ordered list.

This is the perfect start to build your own category menu for your Magento based store. With some small modifications it's possible to only display the childs of the currently active category and attach something like class="active" to the corresponding link within this list.

<?php 
  $helper = $this->helper('catalog/category');
  $categories = $this->getStoreCategories();
  
  function printCategories($categories, $helper) {
    if( count($categories) > 0 ) {
      echo '<ul>';
        foreach( $categories as $category ) {
          echo '<li>';
          echo '  <a href="'.$helper->getCategoryUrl($category).'">'.$category->getName().'</a>';
          if ( count($category) > 0 ) {
            printCategories($category->getChildren(), $helper);
          }
          echo '</li>';
        }
      echo '</ul>';
    }
  }
  
  printCategories($categories, $helper);
?>