Tracking Device

A tracking device is a piece of Applied Phlebotinum that broadcasts its location, and is used to follow the individual or vehicle it's been secretly planted on. Occasionally the individual discovers the device and uses it to lead the trackers on a Wild Goose Chase.

Tracking devices can also be intentionally used by characters on the same side to keep tabs on each other, so that if somebody gets kidnapped, his friends will immediately know where he is. Of course, a Genre Savvy bad guy who happens to find a tracking device used for this purpose could easily use it as a Red Herring.

Most tracking or surveillance devices depicted in fiction are so small that they cannot include important components such as a power source, antenna, microphones, etc. This used to seem like magical tech back in the day, but as Technology Marches On with the advent of mobile phones that allow tracking by cell tower, and USB Pen Drive-sized GPS trackers, extremely miniature tracking devices are rapidly becoming Truth in Television. Indeed, any tracking device that isn't miniscule these days probably qualifies as an Incredibly Obvious Bug.

Examples of Tracking Device include:

Anime & Manga

Early in Magical Girl Lyrical Nanoha ViVid, after Hegemon Ingvalt walked away in apparent victory, the near-immobile Nove Nakajima contacts Subaru Nakajima and reveals that she planted a sensor on her during their battle, so they can track her down and capture her at any time they want.
Both the heroes and villains use these in Science Ninja Team Gatchaman. Finding or evading them are plot points in several episodes.
Ai no Kusabi has "Pet Rings" which all have built in tracers to track a pet's whereabouts. One pet in particular finds this out the hard way.
Ghost in the Shell has tracking devices which can be fired out of pistols. In both the movie and the show, Togusa fires one out of his revolver to track a car.

Comic Books

Often used by Batman.
Also commonly used by Spider-Man.
- It was a Spider-Man comic that led to the invention of electronic tagging.
Dreadstar team late in their fight against Instrumentality suddenly became very unlucky. Though they suspected a mind-controlled spy at first.

Film

The Millennium Falcon gets one in Star Wars, leading the Death Star to the rebel base at Yavin.
- Jango Fett's ship also gets one on it in Episode II.
Used to hilarious effect in The Naked Gun, where Nordberg fails to attach the tracking device, rolls down a hill, and ends up underneath Drebin's own car- "we're right on top of him now!"
The Avengers 1998. Steed gives Mrs. Peel a set of boots with an implanted "micro-tag" (radio homing signal). He later uses it to locate and rescue her.
Johnny Five from Short Circuit has a tracking device that he tosses into the bed of a passing pickup truck.
In The Matrix, the three Agents implant a literal (robotic) bug in Neo's body that they plan to use to track him.
Red Dawn. The Soviets capture one of the American guerillas and force him to swallow the tracking device. Unfortunately the spetsnaz team they have following him gets ambushed, and their homing beacon points directly at the traitor.
Brannigan (1975). The police plant a magnetically-attached bug on the vehicle of the mob lawyer used to make the ransom exchange, but he finds it and plants it on another vehicle while stopped in traffic. Unfortunately John Wayne has planted a second bug inside one of the bundles of ransom money.
James Bond plants one in Auric Goldfinger's Rolls-Royce in Goldfinger. Felix Leiter later homes in on the smaller one Bond is carrying to locate the OO agent.
Buckaroo Banzai. There was apparently one of these on the Oscillation Overthruster (or possibly Penny Priddy), as Buckaroo uses a signal detector to find them inside Yoyodyne.

Literature

Frank Herbert's Children of Dune. When Alia sends Buer Agarves on a diplomatic mission to Stilgar, she conceals a homing signal in the new boots she gives him so her forces can follow him and capture Stilgar.
In the Vorkosigan Saga novel Komarr, Miles briefly thinks he'll be able to get a lead on the villains when they steal his Auditor's seal, which has a tracking device embedded in it. Unfortunately, they know about the device, and almost immediately flush the seal down the toilet to serve as a particularly messy Red Herring.
In Robert A. Heinlein's Time Enough for Love, Lazarus Long Time Travels to meet his family when he was a child. Due to a miscalculation, he winds up in 1917 instead of 1920, just in time for the United States to get into World War I. Prior to leaving, he was unknowingly implanted with a tracking device by his family of the future, which comes in really handy when he gets shot in battle and has to be rescued.

Live Action TV

In the Leverage episode "The Stork Job", Hardison reveals when Parker goes off on her own that he put a GPS tracker in her shoes. It's implied that he keeps tabs on the entire team this way all the time.
Used on an episode of Scrubs so JD can track and avoid the Janitor.
May be one of the most used devices in the series Alias.
The title character of Chuck wears one in his wristwatch so (theoretically) his handlers can always find him.
Used over and over again in Veronica Mars in order to track and find nearly anybody.
In addition to their primary purpose as Comm Links, communicators in Star Trek are used as locating devices. Plots frequently demand that they be removed or jammed.
As part of his release agreement with the FBI, Neal Caffrey of White Collar has to wear an ankle tracking bracelet at all times (until his 4-year term is up, anyway). He cannot leave a 2-mile radius of his home (which, fortunately for him, is in Manhattan) except with an FBI escort, and if he tries to remove it without authorization (occasionally given as part of an operation), it sends out a signal.
Often used on NCIS, for the agents themselves to be rescued or just followed, or suspects or criminals.

Video Games

Franziska plants one of these on her incompetent detective partner Gumshoe in Ace Attorney Justice for All, much to Gumshoe's displeasure.
In Mass Effect 2, after you acquire the Reaper IFF chip, it installs a virus into the Normandy's computer that broadcasts its current location. Which results in the kidnapping of all of your crew except Joker. Kinda makes the Paragon choice in the endgame (to destroy the Collector Base rather than reverse-engineer it, seem the right one: if a mere chip made by the Reapers could outsmart the best AI humans could produce, what would an entire base do?
Ghost Trick: Cabanela fires a special bullet into Yomiel's body, which can be tracked by a modified pocket watch.
In Secret Agent Barbie, Barbie has a gadget which is basically tracking perfume.

Web Comics

This strip of Irregular Webcomic.

Western Animation

Played for laughs in an episode of Jackie Chan Adventures. An episode that involves clones of the heroes has Captain Black place a tracking device on one of the Jackies. He starts bragging about before the real Jackie points out that it's on him instead of the escaped clone.
Also in Swat Kats. Hard Drive and Dark Kat stole the Turbo Kat and used it to commit crimes and frame the heroes. They demanded several bags of money. Commander Feral hid a tracking device in one of them. The bad guys throw away a bag (presumably the one with the tracking device) and, using a device to impersonate T-Bone's voice, said "Nice try, Feral!"
1973-74 Superfriends episode "The Planet Splitter". The Super Friends put microdot homing devices on the remaining 100+ carat diamonds so they can be tracked down after they're stolen. At the end of the episode, Wendy puts Wonder dog on a diet, them puts a microdot homing device on one of his favorite steak bones so they can track it when he steals it.

Real Life

Modern cellular phones have GPS receivers in them to provide locations during emergency calls. Some offer tracking services that allow account holders to keep track of their children, or track a lost or stolen phone.
- Sadly, these capabilities are also increasingly used for more nefarious purposes.
There is a story about how former members of the Staasi, being unemployed in the absence of, you know, East Germany formed private detective agencies that specialize in sweeping for bugs that they put in in the first place.