The "Go back" button for Magento. Designed so that if you landed on Magento from external site then button wont be shown.

gistfile1.php

<?php if (($referrer = Mage::app()->getRequest()->getServer('HTTP_REFERER'))): ?>
    <?php $storeUrlParts = parse_url(Mage::getBaseUrl(Mage_Core_Model_Store::URL_TYPE_WEB)); ?>
    <?php $referrerUrlParts = parse_url($referrer); ?>
    <?php if ($storeUrlParts['host'] === $referrerUrlParts['host']): ?>
        <button type="button" title="<?php echo $this->__('Go back to %s', $referrer) ?>" class="button" onclick="this.innerHTML = '<?php echo $this->__('Redirecting...') ?>'; window.location = '<?php echo $referrer ?>';"><?php echo $this->__('Back') ?></button>    
    <?php endif; ?>    
<?php endif; ?>