Magnifying glass

The magnifying glass was a glass that produced a magnified image of an object. Sun rays sent through a magnifying glass could be used to burn things. It was known to humans, Time Lords and Apalapucians. It was usually carried in a handheld tool.

The Doctor
It was carried by the Doctor in several incarnations: first, (PROSE: A Big Hand for the Doctor) second, (TV: The Highlanders et al.) third, (TV: The Sea Devils et al.) fourth, (TV: Genesis of the Daleks et al.) fifth, (TV: Four to Doomsday) tenth, (TV: Partners in Crime et al.) and eleventh. (TV: The Crimson Horror)

The Tenth Doctor used a magnifying glass to examine an Adipose Industries pendant within the TARDIS. (TV: Partners in Crime)

What appeared to be an oversized magnifying glass was found by the Eleventh Doctor, Amy Pond and Rory Williams in the Two Streams Facility on Apalapucia. In actuality, it was a time glass. (TV: The Girl Who Waited)

Weapon
Magnifying glass could amplify the heat from sun rays.

On one occasion, Donna Noble used the Doctor's glass this way against a Vespiform which then went out the window. When it appeared again later, the sight of Donna holding the magnifying glass, despite not being near sunlight, was enough to scare it off. (TV: The Unicorn and the Wasp)

As a boy, Archibald Flint would sometimes amuse himself by burning ants with a magnifying glass. (AUDIO: The Spectre of Lanyon Moor)

Behind the scenes
Graeme Harper used magnifying glasses to achieve the effect of a distorted image of a main character, as a directorial "signature", p.e. in Army of Ghosts, The Unicorn and the Wasp, and Utopia.