Writing to Christians imprisoned for their faith can be a powerful way of encouraging the suffering and even shortening their prison stay. PrisonAlert.com
is a wonderful resource in this regard. It is run by Voice of the Martyrs (founded by Richard Wurmbrandt, who survived torture under the Nazis), and provides information about Christians around the world who have been arrested for practicing their faith, and how to write them. It even provides a handy translation device to ensure your words will be understood by the prisoners (many who are in Asia, Africa, and the Middle East). Although it's a Protestant organization, they do occasionally give news of Catholics imprisoned for their faith.