Popes have resigned in the past, and more than once, despite popular mythology to the contrary, at least including:

• Pope St. Pontian in 235
  

  Pope St. Pontian, resigned in 235 AD

• Pope Silverius in 537
• Pope John XVIII in 1009
• Pope Benedict IX in 1045
• Pope Gregory VI in 1046
• Pope Celestine V in 1294
• Pope Gregory XII in 1415

Several others are not entirely clear whether it was resignation or deposition, including at least Pope Marcellinus in 308 and Pope Liberius in 366.

Plus there were others who are now considered antipopes who resigned, but at the time may not have been so clear who was the legitimate bishop of Rome. Some popes were deposed, others excommunicated. What I remember from history courses was that about 10% did not serve until death (and not all who did died of natural causes).

Modern popes have considered resignation as an option, most famously:

• Pius VII in 1804 prepared a letter of resignation, to be put into effect if he was captured and imprisoned
• Pius XII in 1943, for the same reason
• Paul VI considered retiring at the age of 75, in 1972, to conform to the law that asked the same of all other bishops
• John Paul II said in 1979 that he was open to the same idea, and it is said that he had a conditional document prepared as early as 1989, and again in 2000.
• Benedict XVI told the cardinals in the days after he was elected that he would resign if necessary, and addressed it again in 2010 in his interview with Peter Seewald, Light of the World

Back in 2000, ecclesiologist Fr. Richard McBrien penned an article for the Tablet, asking the question of resignation with respect to the pontificate of John Paul II. His book, Lives of the Popes: The Pontiffs from St. Peter to Benedict XVI, is one of a handful of good, contemporary resources on the topic.

We need to think more broadly than the bishop of Rome. We have seen other patriarchs and heads of churches resign, both within the Catholic Church, and in broader Christendom. All of them in positions that, in virtually all cases, were also considered normally held until death.

Consider just recently:

• The Catholic Coptic Patriarch Antonios I Naguib resigned in January 2013, at age 77.
• The Catholic Chaldean Patriarch Emmanuel III Delly resigned in December 2012, at age 85.
• The Catholic Maronite Patrairch Nasrallah P. Sfeir resigned in March 2011, at age 90.
• Archbishop Rowan Williams of Canterbury resigned, effective December 2012, at age 62.
• Metropolitan Jonah of the Orthodox Church in America resigned in July 2012, at age 53.

So, while it has been almost 600 years since the last bishop of Rome willingly retired, we can and will get used to the idea. It takes remarkable integrity to lead by such strong example.

Retired professors retire for second time