500 Year Diary

The 500 Year Diary was a journal kept by the Doctor.

Further updates included the 900 Year Diary, (TV: Doctor Who) the 1200 Year Diary, (TV: The History of the Doctor) the Two Thousand Year Diary, (TV: The Girl Who Died) and the Five Thousand Year Diary. (GAME: Lost in Time)

History
According to one account, 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 the Hermit, shortly before the Doctor's regeneration into his second form. (PROSE: The Three Paths)

Another account had the Doctor writing in his 500 Year Diary from his stay at 76 Totter's Lane onwards. (PROSE: Extracts from the Doctor's 500 Year Diary) He hid the diary in the lower compartment of the astral map. To the Eighth Doctor's surprise, Susan knew the Doctor's hiding place during their original travels. (AUDIO: The Shoreditch Intervention)

According to a third account, Susan bought the diary on Tiaanamat, and the Doctor didn't start writing in it until after he left her in 2150. (PROSE: Susan's Diary, TV: The Dalek Invasion of Earth)

Following his 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) When facing the CyberNomads on Nerva Beacon, the Fourth Doctor once again consulted his diary for its records on cybermats and a cyber-virus. (AUDIO: Return of the Cybermen)

The Fourth Doctor searched inside the TARDIS' for the Five Hundred Year Diary, in order to find the DNA code of the Daleks, which he previously recorded. He believed that he left it in the Cloister Room. However, Romana I told him that it was in the swimming pool. Once he located the diary, he programmed the Quantum Gateway to erase the nearby Dalek fleet. (AUDIO: The Final Phase)

While on a planet made of confectionery, 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. The Fourth Doctor once used it as evidence to prove that he was never the phoney Second Doctor. (AUDIO: Survivors in Space)

By the final time of his fifth incarnation, the Doctor confessed to Peri Brown he tried keeping a diary, not chronological, but "the trouble with time travel is one never seems to find the time". (TV: The Caves of Androzani)
 * It is disputable if he was referring to the Five Hundred Year Diary.

He was still using the diary when Mel was his companion, referring to a note in it. (PROSE: Uranus)

Before leaving the TARDIS, Ace took his Five Hundred Year Diary. (PROSE: The Death of Art) After this, it is unclear whether or not she gave it back to him or the Doctor had a spare.

By the time he was travelling with Lucie Miller, the Eighth Doctor had the Five Hundred Year Diary in the TARDIS library. He used the book to pinpoint which species of alien was threatening Little Morton. He was able to correctly guess that he was dealing with a Quitoxin. (AUDIO: The Young Lions)

The Eighth Doctor later checked his "secret diary" to confirm his first incarnation's activities on 7 August 1963. He learned that the First Doctor had been relocating the Hand of Omega. (AUDIO: The Shoreditch Intervention) Despite an account suggesting that the Doctor had switched to a Nine Hundred Year Diary by the end of his Seventh incarnation, (TV: Doctor Who) the Tenth Doctor was once seen writing in the Two Hundred Year Diary during his travels with Martha Jones. (COMIC: Agent Provocateur)

An unspecified incarnation of the Doctor had no plans in his Five Hundred Year Diary one day, and so decided to study ventilation shafts. (PROSE: Doctor Who and the Krikkitmen)

Further diaries
The Fourth Doctor provided two volumes of his time logs to search for references to Traken. He hinted he was too busy to still keep track of his travels. (TV: The Keeper of Traken)

The Seventh Doctor owned a Nine Hundred Year Diary shortly before his regeneration into the Eighth Doctor. (TV: Doctor Who)

The Tenth Doctor had a psychic diary written on psychic paper. He explained to Christina that it held everything he saw and felt. The Doctor kept this diary on a desk in the TARDIS's library, but due to a cloud layer in the library's upper stacks, the desk was rained on no matter where it was placed. The Doctor was forced to use a saucepan to catch the rain so it didn't moulder the psychic paper and produce psychic mould and psychic mushrooms. (PROSE: Keeping up with the Joneses)

The Eleventh Doctor kept a journal of the encounters he had with River Song. (TV: The Impossible Astronaut et. al.) He also had a Twelve Hundred Year Diary, which he and Clara read when the Doctor forgot everything about himself. His memories were returned after reading the book. (TV: The History of the Doctor)

The Twelfth Doctor owned a Two Thousand Year Diary which he used to remind himself of the Mire. (TV: The Girl Who Died)

The Thirteenth Doctor kept a psychic prison diary during her imprisonment in the Judoon prison. (PROSE: The Doctor's Prison Diary, Breaking Free!, TV: Revolution of the Daleks) The Thirteenth Doctor also kept a Five Thousand Year Diary which she wanted to record a recollection from an Ice Warrior into. (GAME: Lost in Time)

Other references
Scandrius, a Time Lord who idolised the Doctor, carried a Five Hundred Year Diary of his own which he wrote while briefly travelling with Tegan Jovanka in a TARDIS that he stole. (AUDIO: Time in Office)

While pretending to be an incarnation of the Doctor, Mr Song disguised River Song's diary as a Five Hundred Year Diary. (AUDIO: Signs)

During one of the times River Song had borrowed the TARDIS, she read the Five Hundred Year Diary. When she found herself in the Great Intelligence's takeover of London, she used her memories of the diary to safely navigate around history. (AUDIO: The Web of Time)

Information from invalid sources
Information recorded in the Doctor's Five Hundred Year Diary concerning The Doomsday Contract stated that there was once a man, John Lloyd, who worked with Douglas Adams on The Hitchhiker's Guide to the Galaxy. Not long after, Douglas became a script editor on Doctor Who, which prompted Lloyd to submit a story treatment entitled The Doomsday Contract. Lloyd toiled over many redrafts of the script, but it fell through and never made it to television.

43 years later, the "remarkable" company Big Finish adapted The Doomsday Weapon into a "full cast audio version". It was a massive success. (WC: Tom Baker stars in John Lloyd's lost Doctor Who adventure, The Doomsday Contract)