Magento shell script (place it in /shell) to output all filterable attributes. You can then copy/paste it into robots.txt to effectively stop bots from indexing your layered navigation. See also https://www.byte.nl/blog/zoekmachine-bots-en-serverbelasting/

filterable_attributes.php

<?php
require_once 'abstract.php';

class Mage_Shell_Filterable_Attributes extends Mage_Shell_Abstract
{
    /**
     * Run script
     *
     */
    public function run()
    {
        $attributes = Mage::getResourceModel('catalog/product_attribute_collection')
            ->addIsFilterableFilter();

        foreach ($attributes as $attribute) {
            echo 'Disallow: /*?' . $attribute->getAttributeCode() . "=*\n";
            echo 'Disallow: /*&' . $attribute->getAttributeCode() . "=*\n";
        }
    }

}

$shell = new Mage_Shell_Filterable_Attributes();
$shell->run();

Using the Google Webmaster Tools parameters I can tell Google what the meaning of each and every parameter is. But ... life if bigger then only Google. Bing, Yandex and other search engines are not aware of Google Webmaster Tools parameters. Therefor this script is useful.