The Five Hundred Year Diary was a journal kept by the Doctor.
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. (ST: The Three Paths)
After regenerating, the Doctor got his diary out of his storage chest and began to read it, even while walking through the mercury swamps of Vulcan. (DW: The Power of the Daleks)
The Doctor later checked his diary while on Telos to read up on a previous encounter with Cybermats. (DW: The Tomb of the Cybermen)
While on Earth, the Doctor checked his pockets for his diary, which contained notes he made about the Sontarans. (DW: The Sontaran Experiment)
While on a planet made of confectionary, the Doctor checked the calendar in his diary and learned that it was K9's birthday. (ST: Special Occasions: 1. The Not-So-Sinister Sponge)
Later, the Doctor obtained a Nine Hundred Year Diary. (DW: Doctor Who)