2 define the new block to the left sidebar
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<reference name="left">
    <block type="catalog/navigation" name="catalog.leftnav" template="catalog/navigation/my_left_nav.phtml" />
</refrence>
<block type="catalog/navigation" name="catalog.leftnav" template="catalog/navigation/my_left_nav.phtml" />
<reference name="left">
    <block type="catalog/navigation" name="catalog.leftnav" template="catalog/navigation/my_left_nav.phtml" />
</refrence>
1
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Add the below code in your theme’s catalog XML file like this:

<block type="catalog/navigation" name="catalog.leftnav" template="catalog/navigation/my_left_nav.phtml" />

It should be placed inside the reference name “left“, to show it in the left sidebar. I hope you know all these stuffs. Let’s get to the point. Just copy and paste the code in the my_left_nav.phtml, which should be in your theme’s catalog/navigation folder.

<!-- List all categories and their second level subcategories -->
<div class="block block-list block-categories">
    <div id="block-categories" class="block-title active">
        <strong><span>Categories </span></strong>
    </div>

<div id="leftnav" class="block-content" style="display:block">
    <?php $helper = $this->helper('catalog/category') ?>
    <?php $categories = $this->getStoreCategories() ?>
    <?php if (count($categories) > 0): ?>
        <ul id="leftnav-tree" class="level0">
            <?php foreach($categories as $category): ?>
                <li class="level0<?php if ($this->isCategoryActive($category)): ?> active<?php endif; ?>">
                    <a href="<?php echo $helper->getCategoryUrl($category) ?>"><span><?php echo $this->escapeHtml($category->getName()) ?></span></a>
                    <?php //if ($this->isCategoryActive($category)): ?>
                        <?php $subcategories = $category->getChildren() ?>
                        <?php if (count($subcategories) > 0): ?>
                            <ul id="leftnav-tree-<?php echo $category->getId() ?>" class="level1">
                                <?php foreach($subcategories as $subcategory): ?>
                                    <li class="level1<?php if ($this->isCategoryActive($subcategory)): ?> active<?php endif; ?>">
                                        <a href="<?php echo $helper->getCategoryUrl($subcategory) ?>"><?php echo $this->escapeHtml(trim($subcategory->getName(), '- ')) ?></a>
                                         <?php $secondLevelSubcategories = $subcategory->getChildren() ?>
                                         <?php if (count($secondLevelSubcategories ) > 0): ?>
                            <ul id="leftnav-tree-<?php echo $subcategory->getId() ?>" class="level2">
                                <?php foreach($secondLevelSubcategories as $secondLevelSubcategory ): ?>
                                    <li class="level2<?php if ($this->isCategoryActive($secondLevelSubcategory )): ?> active<?php endif; ?>">
                                        <a href="<?php echo $helper->getCategoryUrl($secondLevelSubcategory ) ?>"><?php echo $this->escapeHtml(trim($secondLevelSubcategory ->getName(), '- ')) ?></a>
                                    </li>
                                    <?php endforeach; ?>
                            </ul>
                            <script type="text/javascript">decorateList('leftnav-tree-<?php echo $category->getId() ?>', 'recursive')</script>
                        <?php endif; ?>
                                <?php endforeach; ?>
                            </ul>
                            <script type="text/javascript">decorateList('leftnav-tree-<?php echo $category->getId() ?>', 'recursive')</script>
                        <?php endif; ?>
                    <?php //endif; ?>
                </li>
            <?php endforeach; ?>
        </ul>
        <script type="text/javascript">decorateList('leftnav-tree', 'recursive')</script>
    <?php endif; ?>
</div>
</div>

Hope it will help you.