Add attribute column to admin product grid

There is already one flexible module in github for managing grid columns of any entities(orders, products, customers etc.):
https://github.com/magento-hackathon/GridControl

Found this question using Google, the selected answer works but I just tried out this module and wanted to share. It installs through Magento Connect and works extremely well.

(dead link) http://www.magentocommerce.com/magento-connect/enhanced-admin-grids-editor.html new link: https://github.com/mage-eag/mage-enhanced-admin-grids

Note: A better solution is a module approach, like in the accepted answer.

I used this blog and that works great! [link] - Jelle

Here's the gist of that post:

Copy the contents of: /app/code/core/Mage/Adminhtml/Block/Catalog/Product/Grid.php into a new file (if it doesn't exist): /app/code/local/Mage/Adminhtml/Block/Catalog/Product/Grid.php

Find this:

$this->addColumn('sku',
array(
    'header'=> Mage::helper('catalog')->__('SKU'),
    'width' => '80px',
    'index' => 'sku',
));

Below that, add the attribute you want in your admin product grid:

$this->addColumn('import_price',
array(
    'header'=> Mage::helper('catalog')->__('Import Price'),
    'width' => '150px',
    'index' => 'import_price',
));

Find: $collection = Mage::getModel('catalog/product')->getCollection() within the _prepareCollection() function.

And add:

->addAttributeToSelect('import_price')

The article also goes into detail for adding a select dropdown list as well.

Magento: Add attribute columns in ‘Manage Products’ grid