Coming on the heels of the news that Pope Benedict XVI wil be resigning due to advanced age, it begs the question, have there been Popes who have resigned before?
From Wikipedia (though please confirm as it is Wikipedia, after all):
In 1045, Pope Benedict IX agreed, for financial advantage, to resign the papacy. Pope Gregory VI, who to rid the Church of the scandalous Benedict IX had persuaded him to resign, became his successor. Gregory himself resigned in 1046 because the arrangement he had entered into with Benedict was considered simoniacal; that is, to have been paid for. Gregory’s successor, Pope Clement II, died in 1047 and Benedict IX became Pope again.
The best known example of the resignation of a Pope is that of Pope Celestine V in 1294. After only five months of pontificate, he issued a solemn decree declaring it permissible for a Pope to resign, and then did so himself. He lived two more years as a hermit and was later canonized. The Papal decree that he issued ended any doubt among canonists about the possibility of a valid Papal resignation.
Pope Gregory XII (1406–1415), resigned in order to end the Western Schism, which had 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. Before resigning he formally convened the already existing Council of Constance and authorized it to elect his successor.