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<config>
    <adminhtml>
        <events>
            <adminhtml_block_html_before>
                <observers>
                    <mymodule>
                        <!-- Add column to catalog product grid -->
                        <class>mymodule/adminhtml_observer</class>
                        <method>onBlockHtmlBefore</method>
                    </mymodule>
                </observers>
            </adminhtml_block_html_before>
            <eav_collection_abstract_load_before>
                <observers>
                    <mymodule>
                        <!-- Add column to product list -->
                        <class>mymodule/adminhtml_observer</class>
                        <method>onEavLoadBefore</method>
                    </mymodule>
                </observers>
            </eav_collection_abstract_load_before>
        </events>
    </adminhtml>
</config>

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...');
        }
    }
 
}