Stethoscope

A stethoscope was an instrument that amplified sound, allowing the wearer to better hear the sounds emanating from the site on which the bell of the device was placed. Though typically meant for medical practice — as when Harry Sullivan and Martha Jones examined the Doctor's hearts, (TV: Robot, Smith and Jones) it could be used in other, non-medical situations. The Doctor, for instance, frequently used it to amplify the sound that mechanical or electronic devices made, (TV: Fury from the Deep, The Stolen Earth) or to eavesdrop on a conversation in an adjacent room. (AUDIO: Storm Warning, TV: Partners in Crime) The Doctor found it such a useful device that many of their incarnations, including Second, Fourth, Eighth, Tenth, Eleventh and Thirteenth carried one.

By humans
On Earth, most medical students and doctors had stethoscopes, which they used throughout their careers. (TV: Spearhead from Space, Robot, The Hand of Fear, The Christmas Invasion, Smith and Jones)

Tina's lodger was a medical student. Jackie took the lodger's stethoscope while she was asleep and Rose then used it to check that both of the newly-regenerated Tenth Doctor's hearts were working. When Rose later checked, only one heart was beating. (TV: The Christmas Invasion)

After being checked into the Royal Hope Hospital, the Tenth Doctor was examined by Martha Jones, who used her stethoscope to listen to the Doctor's heart. When she did, she found he had two hearts. (TV: Smith and Jones)

By the Doctor
On several occasions, the Doctor used the stethoscope as a non-medical tool.

The Second Doctor used a stethoscope to listen to the pipes emanating from the Euro Sea Gas plant. He at first thought he detected a heartbeat in the pipes, but dismissed it as the sound of a pump. In fact, he had heard the stirrings of organic life—that of the weed creature. (TV: Fury from the Deep)

The Second Doctor again used a stethoscope to listen for an audio lock in the cell he, Jamie McCrimmon, Zoe Heriot and Lt. Joe Sorba were trapped in. (TV: The Space Pirates)

The Fourth Doctor used a stethoscope to examine Erato's spacecraft. He found it the source of a signal he had received. (TV: The Creature from the Pit)

The Eighth Doctor carried a stethoscope he had "borrowed" from Arthur Conan Doyle, which he needed to return to Doyle before the writer stopped practising medicine. The Doctor used it to eavesdrop whilst aboard the R101. (AUDIO: Storm Warning)

The Tenth Doctor used a stethoscope to listen at the door to the Thames Flood Barrier. (TV: The Runaway Bride)

The Tenth Doctor used a stethoscope to listen at the window of Adipose Industries. (TV: Partners in Crime)

The Tenth Doctor and Donna Noble used his stethoscope to listen to the two hearts of Jenny whilst imprisoned. (TV: The Doctor's Daughter)

The Tenth Doctor also used his stethoscope to listen to the knocking sound on the hull of a shuttle bus during a ride to the Sapphire Waterfall. (TV: Midnight)

When the Earth was taken to the Medusa Cascade, Harriet Jones, Torchwood and Mr Smith all worked together to send a signal to Martha's phone. When it rang, the Tenth Doctor answered to find it only to be a signal, which he then followed by using his stethoscope. (TV: The Stolen Earth)

The Tenth Doctor used a stethoscope to listen to the heartbeat of Jackson Lake. (TV: The Next Doctor)

The Eleventh Doctor used his stethoscope on his TARDIS console, helping him realise the rectifier was "playing up". (TV: The Lodger)

The Eleventh Doctor put a stethoscope around his neck to prove he was a doctor. (PROSE: Shroud of Sorrow)