The Doctor's recorder

During his second incarnation, the Doctor owned a recorder. He often enjoyed playing it on multiple occasions, sometimes as a source of self-amusement, but also to help him think.

He played it several times while on Vulcan (DW: The Power of the Daleks), in a marketplace on the Panjistri homeworld to amuse Lilith (NA: Timewyrm: Apocalypse), while in a Scottish prison cell (DW: The Highlanders), while visiting Atlantis (DW: The Underwater Menace), while visiting an Earth colony (DW: The Macra Terror), while imprisoned by the Daleks (DW: The Evil of the Daleks), while imprisoned in Tibet (DW: The Abominable Snowmen), and while under Robot Yeti guard. (DW: The Web of Fear)

When the Doctor's TARDIS landed in Australia, Jamie made the Doctor leave his recorder in the ship before they went out. (DW: The Enemy of the World)

After escaping from the Land of Fiction, the Doctor realised he left his recorder there, and sent himself back in to retrieve it. While there, he encountered Goth, who asked him to help the Time Lords, and thus sent him to meet his future self. (DWM: Future Imperfect)

When the second and third incarnations of the Doctor were transported to Omega's anti-matter universe, the recorder fell into the TARDIS' force field generator, and was therefore not converted into anti-matter during the trip there. When the Doctors discovered this, they offered it to Omega, and the resulting annihilation seemingly destroyed the renegade Time Lord and his world. In exchange for his sacrifice of the instrument, the Third Doctor promised his earlier self that he would buy him a "thousand" more. (DW: The Three Doctors)

Although his memories of meeting his future self quickly faded, the Doctor did remember that he had lost his recorder, and went to a music shop on Amber Station to get a new one. (DWM: Briefly Noted)

He gave his recorder to Flora Millrace, claiming to have extras. (ST: The Avant Guardian)

The Doctor used his recorder to control giant metal spiders on a jungle planet (TVC: Master of Spiders), and to befriend a herd of mammoths on prehistoric Earth. (TVC: Pursued by the Trods)

Sarah Jane Smith discovered the Doctor's recorder in the TARDIS' secondary control room. (DW: The Masque of Mandragora)

A newly regenerated Doctor tried playing the recorder, but realised it was just not him. (DW: Castrovalva)

According to one account, during his ninth incarnation, the Doctor threw the recorder away on a rubbish tip on Earth while having a clear out of some old things. In this case, however, the instrument was referred to as a flute. (WEB: whoisdoctorwho.co.uk)