$product = Mage::getModel('catalog/product');
$attributes = Mage::getResourceModel('eav/entity_attribute_collection')
              ->setEntityTypeFilter($product->getResource()->getTypeId())
             ->addFieldToFilter('attribute_code', 'manufacturer');

$attribute = $attributes->getFirstItem()->setEntity($product->getResource());
$manufacturers = $attribute->getSource()->getAllOptions(false);

print_r($manufacturers);

Mage::getModel('catalog/product')->getCollection()->addAttributeToSelect('manufacturer')->addAttributeToFilter('category_id', '3')