I want to push all out of stock products at the end of list. I did it for Category List and Category Search Page. What i could not do is to do it for tags page.
For future reference and those who want to do the same i bring my method here:
For Category List i tweaked the \app\code\core\Mage\Catalog\Model\Layer.php file and add this code to getProductCollection() method after $collection = $this->getCurrentCategory()->getProductCollection();
For Category Search page i tweaked the the \app\code\local\Smartwave\Ajaxcatalog\Model\Catalog\Layer.php and add the same code to getProductCollection() method after $collection = Mage::getResourceModel('catalogsearch/fulltext_collection');