Courier

A courier came to the battle once bloody and loud
And found only skin and bones where he once left a crowd

—Remember the Alamo

You, an unimportant background character, need something delivered to or across a barren wasteland, deadly jungle, or simply a dangerous urban environment. The internet, phone, or regular mail service isn't going to cut it. You're going to need something special. And what is that something special?

Why, it's the local Courier! A Courier is essentially a mercantile mailman/woman, delivering messages through cities and towns on foot or other single-person conveyance. In fiction this often comes with some level of danger involved, either from the environment the Courier crosses, the package they're delivering, people who may be after the package they're delivering, or simply through the Courier's own recklessness. Even if there's not an element of danger, there will usually be a tight deadline the parcel must be delivered by, forcing the Courier to bust their hump getting it there on time. Le Parkour or other fancy tricks may be employed to get safely from Point A to Point B (the words "parkour" and "courier" both derive from the French for "to run").

Since it's a romantic spy type of job that still allows cynicism with money, combined with the fact that it's an easy way to bring characters to new places or into contact with interesting people, it's ripe for protagonist-hood, but this is not always the case. Can often turn up even in futuristic settings where you'd imagine advanced technology would make human Couriers obsolete.

See Pony Express Rider for the mounted variety and Unstoppable Mailman for the government-employee version. May involve Deadly Delivery, Shoot the Messenger or You Got Murder.

Examples of Courier include:

Anime and Manga

GetBackers mixes in super powers and takes it to the logical extreme.
This is basically Celty's job on Durarara!!.
The crew of the Black Lagoon call themselves couriers. Smugglers, pirates, and mercenaries would also fit.

Comic Books

The subject of the story "The Courier" in Flight volume five, by Kazu Kibuishi.

Film

The protagonist of the The Transporter series of films.
Final Fantasy VII: Cloud becomes one in Advent Children.
A Norwegian courier shows up in Casablanca carrying a message for Victor Lazlo. Naturally he does the equivalent of saying to an entire nightclub: "Hey I'm an agent of a foreign belligerent. Please come get me. I love the feel of rope around my neck in the morning."

Literature

Johnny Mnemonic may be the Trope Maker.^[context?]
Snow Crash has Y.T., although she often delivers through ~~suburbs~~ burbclaves.
Chevette Washington, the protagonist in William Gibson's Virtual Light
The main character in The City of Ember is excited about being a messenger in the city.
The cover story for Miles Vorkosigan as opposed to his real job as head of the Dendarii. A mistake during the rescue of a real courier is what eventually ends his military career.
In Umberto Eco's Loana, the protagonist remembers reading a fascist children's book about a hero trying to smuggle an important message to Italy's then-colony Abyssinia (Ethiopia). This being a serial novel, in Real Life Abyssinia is liberated from the Italian fascists long before the story ends. And at the end, the oh so secret message delivered essentially boils down to: "Hold out!"
Matty from Messenger by Lois Lowry pretty much embodies this trope, minus the money-making aspect (though he does crave the admiration and prestige that comes with doing a dangerous job).
In The Company Novels book Black Projects, White Knights, Kalugin has a run-in with a brain-damaged immortal literally named Courier, who goes berserk if he spends the night in one place for more than one night in a row.
Fiona from William Gibson's Zero History has this as her regular job. Gibson really loves this trope.
In Terry Pratchett and Neil Gaiman's book Good Omens, a courier is tasked with informing the Horsemen of the Apocalypse that Armageddon is imminent. He tracks down all of them including Death, despite having no apparent powers of his own.
In the Warrior Cats series, apprentices play this role during the battle against the Dark Forest cats - traveling through a battle-filled forest where any enemy will kill them on sight so that the Clans can send messages to each other on the status of their warriors.

Live-Action TV

The job of half the cast of Dark Angel. It's a bike messaging service called Jam Pony that pays minimum wage and has high turnover. Max and Alec particularly like the free sector passes and opportunity to case joints.
While it's just a throwaway gag, Vince in The Mighty Boosh' subscribes to hyper-cutting-edge fashion magazine Cheekbone, which has to be delivered by ninjas to avoid being obsolete by the time it's read.
Because of the way Faster-Than-Light Travel works in Andromeda, messages have to be relayed by couriers, so courier ships are a common sight.

Tabletop Games

Shadowrun: Runners sometimes get hired to do a courier job. One memorable one was to deliver a dragon's egg.
In Iron Crown Enterprise's Cyberspace game, the Skateboys are a gang that carries messages and packages while riding motorized skateboards.
Dying Earth RPG supplement Scaum Valley Gazetteer. The River Skaters use ice skates to carry messages along the frozen Scaum River during winter.
Traveller: Justified in that FTL communications are not possible so messages have to be carried through jump space on a starship before being delivered.

Video Games

Mirror's Edge: Runners, including the player character, have a valuable job for La Résistance Twenty Minutes Into the Future, because it's virtually impossible to send secure messages over the net.
InFAMOUS: Cole is couriering the Ray Sphere when it activates.
Mega Man ZX: Vent and Aille are couriering the Biometals when they activate.
The protagonist of Fallout: New Vegas is The Courier. And the plot revolves around the package (s)he's carrying.
Unlimited Saga has the Carriers' Guild, which Ventus joins at the start of his scenario. Parts of his quest involve him making deliveries; in addition, he can take several optional Side Quests of this nature. While other characters can recruit him during their stories, they still can't access these special quests themselves.
An item in Defense of the Ancients gives you an animal courier that can deliver items to and from your team and your base. Unsurprisingly, it is a prime target of the other team.
In Red Dead Redemption the series of hidden missions at the end of the game has Marsten pretending to be one.
In Transformers: War for Cybertron, Bumblebee is acting as a courier because the communications in Iacon City aren't safe from the Decepticons. Ironically, the one he's trying to get a message to (Optimus Prime) is the one that saves him from a Deception ambush. The message he's delivering (that Zeta Prime is assumed dead) is disturbing emough that Optimus decides to step up and take "temporary" command of the Autobots.

Western Animation

Get Ed is about a whole team of these.

Real Life

The British Queen's Messenger Service is responsible for carrying diplomatic mail. Which can mean anything from, "Call off the plan to assassinate Hitler-He's winning the war for us", to "Can you please send me a better selection of liquor? Every other embassy in this town is embarrassing us."
Any major city in the United States (and other countries) has bicycle messengers. While thanks to email and fax they are no longer as numerous as they were in the 1980s (when they were the fastest way to send a document from, say, Wall Street to Midtown in Manhattan), they still survive, carrying packages and other physical objects that can't be sent electronically but need same-day delivery across a city.