Get Category Tree Model for Adminhtml

Adminhtml_Model_Source_Category_Tree.php

<?php

class Namespace_Adminhtml_Model_Source_Category_Tree extends Mage_Core_Model_Abstract
{
   /**
     * @return array
     * @throws Mage_Core_Exception
     */
    public function toOptionArray()
    {
        /**
         * Get all active categories
         */
        $categoryCollection = Mage::getModel('catalog/category')
            ->getCollection()
            ->addAttributeToSelect('name')
            ->addAttributeToSort('path', 'asc')
            ->addFieldToFilter('is_active', array(
                'eq' => '1'
            ))
            ->load()
            ->toArray();

        /**
         * Build the 'tree'
         */
        $categoryTree = array();
        foreach ($categoryCollection as $categoryId => $categoryData) {
            if (isset($categoryData['name'])) {

                $label = $categoryData['name'];

                for ($i = 1; $i < $categoryData['level']; $i++) {
                    if ($i == 1) {
                        $label = '-- ' . $label;
                    } else {
                        $label = '--' . $label;
                    }
                }

                $categoryTree[] = array(
                    'label' => $label,
                    'level' => $categoryData['level'],
                    'value' => $categoryId
                );
            }
        }

        return $categoryTree;
    }
}

system.xml

<?xml version="1.0" encoding="utf-8" ?>
<config>
    <tabs>
        <namespace_module translate="label" module="namespace_module">
            <label>Custom Label</label>
            <sort_order>110</sort_order>
        </namespace_module>
    </tabs>
    <sections>
        <namespace_module translate="label" module="namespace_module">
            <label>Custom Label</label>
            <tab>namespace_module</tab>
            <frontend_type>text</frontend_type>
            <sort_order>100</sort_order>
            <show_in_default>1</show_in_default>
            <show_in_website>1</show_in_website>
            <show_in_store>1</show_in_store>
            <groups>
              <category translate="label">
                    <label>Categories Configuration</label>
                    <sort_order>2</sort_order>
                    <show_in_default>1</show_in_default>
                    <show_in_website>1</show_in_website>
                    <show_in_store>1</show_in_store>
                    <fields>
                        <custom_tag>
                            <label>Custom Label</label>
                            <frontend_type>select</frontend_type>
                            <source_model>namespace_adminhtml/source_category_tree</source_model>
                            <sort_order>10</sort_order>
                            <show_in_default>1</show_in_default>
                            <show_in_website>1</show_in_website>
                            <show_in_store>1</show_in_store>
                        </custom_tag>
                    </fields>
              </category>  
            </groups>
        </namespace_module>
    </section>
</config>