500 Year Diary

The Five Hundred Year Diary was a journal kept by the Doctor.

History
The First Doctor began writing the diary whilst still on Gallifrey, but left it there when escaping with Susan. It was returned to him by his mentor K'anpo Rimpoche, shortly before his first regeneration. (PROSE: The Three Paths)

Following this regeneration, the Second Doctor got his diary out of his storage chest and began to read it, even while walking through the mercury swamps of Vulcan. (TV: The Power of the Daleks)

The Doctor later checked his diary while on Telos to read up on a previous encounter with Cybermats. (TV: The Tomb of the Cybermen)

While on Earth, the Fourth Doctor checked his pockets for his diary, which contained notes he had made about the Sontarans. (TV: The Sontaran Experiment)

While on a planet made of confectionary, the Fourth Doctor checked the calendar in his diary and learned that it was K9's birthday. (PROSE: The Not-So-Sinister Sponge)

The Doctor would sometimes use his diary to check his history. He once used it as evidence to prove that he was never the phony Second Doctor. (AUDIO: Survivors in Space)

Before leaving the TARDIS, Ace took his Five Hundred Year Diary. (PROSE: The Death of Art)

Other diaries
The Seventh Doctor replaced this diary with a Nine Hundred Year Diary shortly before his regeneration into the Eighth Doctor. (TV: Doctor Who)

The Eleventh Doctor kept a journal of the encounters he had with River Song. (TV: The Impossible Astronaut)