Under the noraml conditions (sacramental communion, confession 3 weeks before or after, prayers for the intentions of the Holy Father, and detatchment from all affection toward sin), the recitation of the "Prayer before a Crucifix" gains a plenary indulgence on Fridays during Lent.
"Look down upon me, good and gentle Jesus, while before Thy face I humbly kneel and, with burning soul, pray and beseech Thee to fix deep in my heart lively sentiments of faith, hope and charity; true contrition for my sins, and a firm purpose of amendment. While I contemplate, with great love and tender pity, Thy five most precious wounds, pondering over them within me and calling to mind the words which David, Thy prophet, said of Thee, my Jesus: They have pierced My hands and My feet, they have numbered all My bones. Amen."
The prayer can be found in the red misallette at Mass on pg. 65