Plot Device

A plot device is an object or character in the story whose purpose is purely to drive the Plot, maintain its flow, or resolve situations within it. It could be something everybody wants to obtain, a device that must be destroyed, or an annoying teenager who must be protected at all costs.

It may also be an object or gadget introduced early in the story for the sole purpose of solving a sticky situation later on.

The term is commonly used in a derisory manner, on the grounds that the best stories are character driven, and using an object to make things happen is thus seen as a sign of bad writing. However, there are plenty of good stories which do indeed revolve around a plot device; equally, a plot device can very easily be used to generate conflict and thus spark a character-driven story.

Whenever someone uses the term derisively (if they are using the term properly), it is because there is a dissonance between it and the actual needs of the story. For example, the hero has a chance to stop the Big Bad but he is called away because his wife is Trapped by Mountain Lions. It makes the story more complicated without adding anything in return. The villain doesn't build any credentials by setting up the Sadistic Choice and the hero doesn't spend time agonizing over his missed opportunity.

Subtropes:

Amplifier Artifact
Artifact of Attraction: An item which magically causes everybody to Lust after it, generating conflict.
Artifact of Death
Artifact of Doom: A powerful item that causes corruption in all who use it.
Chekhov's Gun
Convenient Eclipse
Death Trap
Deus Ex Machina: A plot device that gets the heroes out of danger.
Diabolus Ex Machina: A plot device that gets the heroes into danger.
Plot Coupons: A set of items which the characters all want to collect.
MacGuffin: A plot device which nobody actually uses, and whose nature and identity are basically irrelevant.
Magnetic Plot Device: A plot device whose very presence causes things to happen.
Suspiciously Specific Sermon
Translator Microbes: A plot device that lets everybody communicate clearly.

Lampshade Hangings and Other References:

Almost by definition, stories have plot devices. Examples should be limited to lampshade hangings and references to the term.

Comic Books

One Captain Mar-Vell comic, starring Genis-Vell, featured a very clever Kree fellow named "Plaht," whose "device" cleared away several plot inconveniences.

Tabletop Games

In The Dresden Files RPG, certain characters are noted as being "plot device level" characters. Most of them are considered Plot Devices because they're so ridiculously powerful that no Player Party would ever reasonably be able to take them on directly, making stats meaningless, and thus would have to outmanuever, work around, or run away from them; these include the Faerie Queens, Angels, and Dragons. A handful, however, are noted as being a Plot Device because they're so far down on the other side of the scale that they can only really provide one specific and specialized purpose, such as Elidee, a tiny pixie—even by pixie standards—who shows up briefly in one book to serve as a guide/flashlight for Dresden.

Video Games

One of the various MacGuffins that City of Heroes radio missions will occasionally ask you to retrieve is a "P.L.O.T. Device." Its description:

The Phased Linear Oscillation Transducer is a miraculous device, capable of producing such a variety of effects that many find it simply unbelievable. However, overusing a P.L.O.T. device can have serious consequences, and the more Egregious uses can strain the very fabric of reality. P.L.O.T devices have fallen out of favor overall, but many a young and reckless pioneer has picked up a well-used P.L.O.T. device and run with it.

Web Comics

Mr. Mighty of Everyday Heroes realizes that he can subject Matt O'Morph to a bit of Harmless Freezing with a carbon-dioxide fire extinguisher ... which has a label on the side that reads, "Warning, Plot Device!"
A literal plot device can be seen here in this Cyanide & Happiness strip.
In Nip and Tuck, when a character complains of an obvious plot device in the Show Within a Show, someone else informs him it was Real Life.

Web Original

Dr. Bill Ernoehazy, an Orion's Arm contributor, wrote another story with a Faster-Than-Light Travel system called the Plott-deVice drive.
This short by Seth Worley features a device called Plot Device, which is a yellow box with large friendly letters reading "Plot Device" with a Big Red Button on its top and enables the user to turn his life in a movie and travel between different film genres.

Western Animation

There is a character in Sheep in The Big City, literally called The Plot Device, which is a robot that can cause nearly anything to happen to advance the plot.