According to many (but not all) faithful Catholics, the Virgin Mary appeared to children at Fatima in Portugal and, among other things, revealed certain prayers.
The Fatima prayer is suggested by the Church to be used when praying the rosary (at the end of each decade):
O my Jesus, forgive us our sins, save us from the fires of hell, lead all souls to Heaven, especially those who have most need of your mercy.
Another prayer given by Our Lady of Fatima:
My God, I believe, I adore, I hope, and I love You. I beg pardon of You for those who do not believe, do not adore, do not hope, and do not love You.