All of the original ancient and medieval artwork of Saint Peter's was lost in the reconstruction. Entire mosaic cycles dating back to the 4th and 5th centuries were destroyed. But, a few Renaissance and Early Renaissance works (including Michelangelo's Pieta) were salvaged from the old church.
Giotto, The Stefaneschi Altarpiece, front
Giotto, The Stefaneschi Altarpiece, back
This was commissioned by Cardinal Jacopo Stefaneschi as part of an effort to glorify Saint Peter's in order to bring back the papacy from its "exile" in Avignon. It was once thought to have stood on the high altar. Now it is thought to have stood on the Canon's Altar nearby.
Copy by Francesco Beretta of Giotto's Navicella (Christ Walking on the Water)
A 17th century copy of Giotto's Navicella mosaic in the narthex of Saint Peter's today.
Cardinal Jacopo Stefaneschi (a cousin of Enrico Scrovegni, Giotto's patron for the frescoes in the Arena Chapel in Padua) kept Giotto very busy in Rome. He commissioned Giotto to make an enormous outdoor mosaic facing the atrium courtyard in front of Saint Peter's. The mosaic showing Christ walking on water and Peter coming out to meet him, was always known as the "navicella," the little ship. It was once Giotto's most famous and celebrated work. Now, there is almost nothing left of it except the copies you see above.
Arnolfo di Cambio, the Florentine sculptor and architect made the famous statue of Saint Peter with the worn feet.
Here is the statue vested with people touching the feet, as they have done for centuries.
Here is Antonio Polaiuolo's magnificent bronze tomb for Pope Sixtus IV, the builder of the Sistine Chapel, and uncle of Pope Julius II who decided to rebuild Saint Peter's.