The Holy Father, Pope Benedict XVI, continues his catechesis on prayer this week by turning to the Old testament and the prayer of Abraham. Abraham uses intercessory prayer to ask God not to destroy the city of Sodom. The Holy Father points out that by doing this, Abraham appeals to God's justice, but also to his mercy.
Praying for others is perhaps the most important gift we can give. Scripture, along with the Communion of Saints, provide the benefits and graces of this type of prayer. Abraham, our father in faith, is one of the very first examples of this type of prayer.
You can read Pope Benedict's catechesis here.