Pope Benedict IX
In 1045, Pope Benedict IX resigned the papacy. Pope Gregory VI became his successor. Gregory himself resigned in 1046. Gregory's successor, Pope Clement II, died in 1047 and Benedict IX became Pope again.
Pope Gregory VI
Pope Gregory VI stepped down in 1046.
Pope Celestine V
In 1294, Pope Celestine V resigned after only five months of being Pope. He issued a solemn decree declaring it permissible for a Pope to resign, and then did so himself.
Pope Gregory XII
Pope Gregory XII resigned in 1415 in order to end the Western Schism. The Western Schism reached the point where there were three claimants to the Papal throne, Roman Pope Gregory XII, Avignon Antipope Benedict XIII, and Pisan Antipope John XXIII.