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app/code/local/My/Module/etc/config.xml

    
        
            
                
                    
                        
                        mymodule/adminhtml_observer
                        onBlockHtmlBefore
                    
                
            
            
                
                    
                        
                        mymodule/adminhtml_observer
                        onEavLoadBefore
                    
                
            
        
    

app/code/local/My/Module/Model/Adminhtml/Observer

class My_Module_Model_Adminhtml_Observer
{

    public function onBlockHtmlBefore(Varien_Event_Observer $observer) {
        $block = $observer->getBlock();
        if (!isset($block)) return;

        switch ($block->getType()) {
            case 'adminhtml/catalog_product_grid':
                /* @var $block Mage_Adminhtml_Block_Catalog_Product_Grid */
                $block->addColumn('COLUMN_ID', array(
                    'header' => Mage::helper('mymodule')->__('COLUMN HEADER'),
                    'index'  => 'COLUMN_ID',
                ));
                break;
        }
    }

    public function onEavLoadBefore(Varien_Event_Observer $observer) {
        $collection = $observer->getCollection();
        if (!isset($collection)) return;

        if (is_a($collection, 'Mage_Catalog_Model_Resource_Eav_Mysql4_Product_Collection')) {
            /* @var $collection Mage_Catalog_Model_Resource_Eav_Mysql4_Product_Collection */
            // Manipulate $collection here to add a COLUMN_ID column
            $collection->addExpressionAttributeToSelect('COLUMN_ID', '...Some SQL goes here...');
        }
    }

}

This come from Clockworkgeek user on a stackoverflow question. This is gold!