The manipulator arm was a long, thin, mechanical arm found on all Dalek models except for the Emperors, the Time War Emperor's guards and special weapons models. The manipulator arm was found in the right-hand socket in the Dalek midsection, with the gunstick on its left. Its function varied depending on its design, and it could be swapped out for a tool more specific to the Dalek's current needs.
The ends of the manipulator arms typically were shaped like a plunger. For this reason, Dalek technology often had circular controls which the Daleks could arm or disarm by rotating. The black tip at the end, which gave the arm its "plunger" shape, could alter its form to better interact with specific objects, for example, finding the combination of electronically-locked doors. (TV: Dalek)
Even if the Dalek died, its arm could still operate machinery for a live person (TV: Remembrance of the Daleks). The arm also had a telescopic feature to to extend its length. (TV: The Dalek Invasion of Earth, Dalek)
It could also serve as a weapon; with the tip placed over the nose and mouth, the arm could generate suction to suffocate and kill a human and enough traction to crush a human skull or rip through sheet metal. (TV: Dalek, TV: Doomsday, PROSE: I am a Dalek) The Tenth Doctor claimed to have seen one crush a brick with ease. (PROSE: Prisoner of the Daleks)
At least in the case of the Cult of Skaro, the arm could also scan brainwaves for information or act as a quick intelligence scan. (TV: Doomsday, Daleks in Manhattan) However, if a large amount of information needed to be analysed, it could be drained out by force by encasing a subject's head inside their "plunger" tips, which would stretch the victim's head and brain to wring out their knowledge through information probes. The probes were strong enough to have a deadly effect on humans, draining them so fiercely that their bodies shrivelled up and became burned out husks. (TV: Doomsday)
Referred to by Daleks as pyro-flames, these were small cylindrical pieces at the end of the arm which shot a stream of fire to clear swaths of forest. (TV: The Daleks' Master Plan) The Daleks later used other flame throwers, which they referred to as flame guns, to melt through ice when searching for "the Abomination". The ends resembled the Progenitor Daleks' eyestalks. (COMIC: The Only Good Dalek) Daleks in First World War used flamethrowers to clear forests to pursue the Doctor and his companions. The flames came from their arms. (COMIC: The Dalek Project)
Seismic detector (or perceptor)A small device which looked like a compass suspended on gimbals, the seismic detector was employed to locate enemy time machines outside the vortex. It could also detect approaching life-forms or the paths of fleeing ones, even through sealed doors. (TV: The Chase)
Electrode unitThe electrode unit was a device which looked like a radar dish. It could operate electronic controls remotely. It was used in this manner to enter the Mechanoid City. (TV: The Chase)
SyringeOne of the members of the Cult of Skaro had an alternative attachment - a syringe. The syringe was fastened directly to the end of the manipulator arm in place of the plunger. Its possible other scientist or surgeon Daleks used these devices (TV: Daleks in Manhattan)
To cut through doors, early Daleks used a long, thin instrument with two spheres and a clear disc set midway up. (TV: The Daleks) Assault Daleks had a claw and blowtorch combination. These were used for cutting through strong metals and for picking up objects normal daleks couldn't. (TV: The Parting of the Ways, PROSE: Prisoner of the Daleks)
One alternative weapon was seen on two of the Emperor's Personal Guard. It seemed to be an upgraded Dalek gunstick, but significanty larger, and with a large sphere on the end. The other side still possessed a gun. Another of the Dalek Emperor's guards had an arm with forward-jutting silver protrusions on a small ball. (TV: The Parting of the Ways)
Behind the scenes
One Battles in Time trading card features a Dalek with a buzz-saw attachment. This is only seen in the Doctor Who book Prisoner of the Daleks, where a Dalek tries to use the buzz-saw to remove Space Major Jon Bowman's brain. In some magazines these Daleks are shown to have a black skirt.