Dowsing Device

An object, such as a forked branch or wooden stick, used to locate hidden objects by consulting supernatural or magical forces. The Dowsing Device itself is not necessarily magical, but it functions as a focus at least.

Another variation is to use a pendulum, either by carrying it and following the direction of its movement or by holding it over a map.

This is a very old "folk magic" idea in the real world. In fiction, it tends to be used either as Functional Magic or as something for the skeptic to snark at - or sometimes, both at the same time.

Examples of Dowsing Device include:

Anime and Manga

James uses one in an episode of Pokémon.
Numata of Kurosagi Corpse Delivery Service normally uses a pendulum to dowse for corpses. In one story, however, he uses the more traditional bent sticks to honor the man who taught him the technique, who's been murdered himself.
JK's Iconic Item in Seikimatsu Occult Gakuin. He can also use them as weapons, lockpicks and pretty much anything else.
Eila uses one in Strike Witches 2, episode seven, to track down the Neuroi that's been invading the girls' pants.
Kurapika in Hunter X Hunter uses one of his nen chains to locate people among others.

Comic Books

In Tintin, Professor Calculus occasionally attempts dowsing with a pendulum. Its success varies, but it generally finds something he's looking for but doesn't know he's looking for.

Film

In The Princess Bride, Inigo prays to his late father to guide his sword—then successfully divines the entrance to the Pit Of Despair.
Coraline (at least in the movie version) uses one that turns out to be made of poison oak.
The Thing That Couldn't Die (a horror film which would have languished in well-deserved obscurity had Mike and the Bots not discovered it) centers around a young girl who can 'water-witch' and consequently discovers the body of a man (justly) condemned to a Fate Worse Than Death.

Live Action TV

The Munsters: Grandpa Munster had a transistorized one with multiple settings.

Grandpa: Ah! Here it is! cackles with glee My radio direction finder.
Herman: Direction finder? Grandpa, that's just an old-fashioned divining rod!
Grandpa: True, true, heh, but I had it transistorized last month! Dials show that the direction finder can be set to find Water, Girls, Metal and Flying Objects.

Gilligan used a divining rod in an episode of Gilligan's Island.

Newspaper Comics

Modesty Blaise recurring character Dinah Collier is an effective pendulum dowser (and blind).

Video Games

Pokémon has the Itemfinder, also called the Dowsing Machine in Japan and, internationally, from Generation IV onwards. The Pokétch, introduced in Pokémon Diamond Pearl and Platinum, features a Dowsing Machine application which essentially performs the same function.
In the first Broken Sword game, George briefly uses a divining rod to find an ancient well (he finds nothing but an extremely old tin can, but when he tosses it aside...).
Persona 4: Ms Sofue has one. One sidequest requires the MC to get special materials for her to build a new one once her old one has "stopped responding to her powers".
In Touhou, dowsing is Nazrin's schtick. Her rods appear to be weather vanes. Also uses a crystal pendant.
The Legend of Zelda: Skyward Sword has this as a main mechanic, to search for keys, pick-ups and Zelda.

Western Animation

The Superhero Squad Show episode "Mysterious Mayhem at Mutant High": The Ringmaster has a divining rod that can find fractals.
Used in the first episode of Total Drama World Tour.
In one Looney Tunes short, Wile E. Coyote gets his tail on fire, so he uses a divining rod to find for water to douse it.
In the Ren and Stimpy episode "Royal Canadian Kilted Yaksmen", one is used to find a vein of Canada's greatest resource: dirt. It consists of a shovel with some fruit placed on top.

Real Life

Dowsing is Older Than Steam, as seen here.
Still in use, as seen in this ad and this video.
There is a theory behind dowsing based on the change in gravity over large underground gaps or caves (like a subterranean water pocket compared to surrounding rock), which could be felt in the pendulum (either fork, rod, or actual pendulum, the important thing is it moves in a pendulum like fashion) by a highly trained and very sensitive operator. However, he or she should be sensitive as hell to feel such an infinitesimal change in oscillation by hands only.