In the 2,000-year-old history of the Catholic Church, only three other Popes besides Benedict XVI have voluntarily resigned their positions.
The first Pope recognized by the worldwide Church as having resigned by free will is Saint Clement I, who led the early Christian Church in the First Century AD.
In 1294, Celestine V was chosen after a conclave that lasted three years. However, just six months into his pontificate he announced his intention to resign because he considered himself unsuitable for the position. Until his election, he had been a hermit. His resignation gave way to the law within the Catholic Church which allows Popes to resign of their own free will.
The third Pope to announce his intention to leave the Chair of Peter was Gregory XII in 1415. His resignation is seen as a sacrificial gesture to heal the Great Western Schism at the time.