Example - add custom columns to the Magento administration Catalog > Product grid

Grid.php

<?php
class Zyn_Common_Block_Adminhtml_Catalog_Product_Grid extends Mage_Adminhtml_Block_Catalog_Product_Grid {

    protected function _prepareColumns() {	
        
	$this->addColumn('zyn_featured',
        array(
            'header'=> 'Show on Homepage',
            'width' => '50px',
			'index' => 'zyn_featured',
			'type'  => 'options',
			'options' => array(0 => 'No', 1 => 'Yes')
    	));

        parent::_prepareColumns();

    }

    protected function _prepareCollection() {
	$store = $this->_getStore();
        $collection = Mage::getModel('catalog/product')->getCollection()
            ->addAttributeToSelect('sku')
            ->addAttributeToSelect('name')
            ->addAttributeToSelect('attribute_set_id')
            ->addAttributeToSelect('type_id')
            ->addAttributeToSelect('zyn_featured');

        if (Mage::helper('catalog')->isModuleEnabled('Mage_CatalogInventory')) {
            $collection->joinField('qty',
                'cataloginventory/stock_item',
                'qty',
                'product_id=entity_id',
                '{{table}}.stock_id=1',
                'left');
        }
        if ($store->getId()) {
            $adminStore = Mage_Core_Model_App::ADMIN_STORE_ID;
            $collection->addStoreFilter($store);
            $collection->joinAttribute(
                'name',
                'catalog_product/name',
                'entity_id',
                null,
                'inner',
                $adminStore
            );
            $collection->joinAttribute(
                'custom_name',
                'catalog_product/name',
                'entity_id',
                null,
                'inner',
                $store->getId()
            );
            $collection->joinAttribute(
                'status',
                'catalog_product/status',
                'entity_id',
                null,
                'inner',
                $store->getId()
            );
            $collection->joinAttribute(
                'visibility',
                'catalog_product/visibility',
                'entity_id',
                null,
                'inner',
                $store->getId()
            );
            $collection->joinAttribute(
                'price',
                'catalog_product/price',
                'entity_id',
                null,
                'left',
                $store->getId()
            );
        }
        else {
            $collection->addAttributeToSelect('price');
            $collection->joinAttribute('status', 'catalog_product/status', 'entity_id', null, 'inner');
            $collection->joinAttribute('visibility', 'catalog_product/visibility', 'entity_id', null, 'inner');
        }

        $this->setCollection($collection);

        //parent::_prepareCollection();
        $this->getCollection()->addWebsiteNamesToResult();
        return $this;
    }
}