Magento: Get a custom attribute value without loading the entire product

I just want to improve @JMTyler answer, because I found out you don't need a real product model to get the getResource()

So you can just do it having a product id and using a singleton ( this would be better in case you are doing it in a loop so you don't actually create the model many time )

$product_id = 10075;
$_resource = Mage::getSingleton('catalog/product')->getResource();
$optionValue = $_resource->getAttributeRawValue($product_id,  [ATTRIBUTE_ID/ATTRIBUTE_CODE], Mage::app()->getStore());
echo $optionValue;

Other simple way is add this "custom_attribute" to the list of attributes to get by default when you check product data from a quote item.

If you already created a custom module in config.xml add this.

<global>
    ...
    <sales>
        <quote>
            <item>
                <product_attributes>
                    <custom_attribute />
                </product_attributes>
            </item>
        </quote>
    </sales>
    ...
</global>

This depends on which version of Magento you're running. Different versions have different offerings. If you're running Community edition 1.6+, the Catalog module has a very nice method just for you!

Try the following:

$_item = $this->getProduct()->getId();
$_resource = $this->getProduct()->getResource();
$optionValue = $_resource->getAttributeRawValue($_item, 'custom_attribute_value', Mage::app()->getStore());
echo $optionvalue;

If you're interested, you could dive down into Mage_Catalog_Model_Resource_Abstract to see what this little guy is doing. It's essentially just a query (admittedly a rather complex one, as EAV tends to be) to retrieve the one attribute you asked for (or the multiple attributes you asked for, since you can pass an array as well).