If you don’t like the fact that the left column shows subcategories when you click on a parent category, but it stays in the parent category, go to your view.phtml file in app/design/frontend/default/your_template/template/catalog/layer/view.phtml and change the code to this:
<?php
/**
* Category layered navigation
*
* @see Mage_Catalog_Block_Layer_View
*/
?>
<?php if($this->canShowBlock()): ?>
<div class=”box layered-nav”>
<div class=”head”>
<h3><?php echo $this->__(’Shop by’) ?></h3>
</div>
<div class=”border-creator”>
<?php echo $this->getStateHtml() ?>
<?php if($this->canShowOptions()): ?>
<div class=”narrow-by”>
<h4><?php echo $this->__(’Browsing Options’) ?></h4>
<dl id=”narrow-by-list”>
<?php $_filters = $this->getFilters() ?>
<?php foreach ($_filters as $_filter): ?>
<?php if($_filter->getItemsCount()): ?>
<?php if($_filter->getName() != “Category”){ ?>
<dt><?php echo $this->__($_filter->getName()) ?></dt>
<dd>
<?php echo $_filter->getHtml() ?>
</dd>
<?php } endif; ?>
<?php endforeach; ?>
</dl>
<script type=”text/javascript”>decorateDataList(’narrow-by-list’)</script>
</div>
<?php endif; ?>
</div>
</div>
<?php endif; ?>
<!– [ends] .browse-by // –>
Save the file and that’s it.
NOTE: if you want to keep the category option in the search results (and you should), create a new view.phtml file with a different name, do a different layout file (2-columns-search.phtml for example) to call that new view-search.phtml file, then call that file from your catelogsearch.xml file.