Get all possible value (dropdown options) of a product attribute in Magento.

all_attribute_options

<?php
  //Possible color value
  $attribute = Mage::getModel('eav/config')->getAttribute('catalog_product', 'color'); //"color" is the attribute_code
  $allOptions = $attribute->getSource()->getAllOptions(true, true);
  foreach ($allOptions as $instance) {
    $id = $instance['value']; //id of the option
    $value = $instance['label']; //Label of the option
  }
?>