Ramscoop

One of the most promising methods of propulsion for spacecraft, particularly spacecraft that need to cover vast interplanetary or interstellar distances, is nuclear fusion. The fusion of hydrogen into helium produces more than two million times as much energy as chemically combusting the same quantity of hydrogen with oxygen.

But even with such monumental fuel efficiency, a fusion-powered rocket ship still has to carry all of its hydrogen fuel along with it. It can only go so fast before it runs out of gas. If you add bigger fuel tanks, you quickly run into a problem: You're now having to burn more fuel just to push your extra unburned fuel along. This is called the mass ratio problem, and it adds up quickly. Even if you pack your space ship to the gills with hydrogen, so that 95% of its total mass is fusion fuel, and your fusion engines are 100% efficient, you'll still run your tanks dry by the time you reach a paltry 35% of the speed of light. And then you won't have any fuel left to slow down at the end of your trip.

But ... the space between the planets and stars is filled with hydrogen! Not much hydrogen, to be sure -- current estimates for the local interstellar medium come in at about 1 atom of hydrogen for every 10 cubic centimeters of space -- but if you could build a friggin' ginormous scoop on the front of your space ship, it might be possible to scoop in this tenuous interstellar hydrogen and use it to run your fusion engines indefinitely. You'd have an interstellar ramjet that would never run out of fuel.

This is also known as a Bussard Collector, after the guy who first proposed it in 1960.

In Real Life, no nuclear fusion technology (aside from hydrogen bombs) has yet "broken even" -- they all consume more energy to induce nuclear fusion than they get out of it. And, worse, the most promising fusion technologies don't use the regular garden-variety hydrogen lying around the universe, they use a much rarer isotope called 2H or deuterium. As explained over on The Other Wiki and Atomic Rockets, since Bussard first proposed the idea there's been some serious questions about whether it would actually work. Still, a lot of fiction features them as that news hasn't reached everyone yet.

For some for whom the news has reached them yet, three compromises to Bussard's design have cropped up:
 * 1) The Ram-Augmented Interstellar Rocket, or RAIR. This starship uses the Bussard scoop to collect the interstellar medium not as fuel, but as simple reaction mass, i.e. as material to throw out the back of the spacecraft. The spacecraft still has to carry its own fuel supply, but the burning of that fuel can now accelerate a much larger amount of material than what the space ship carries on board. This increases the efficiency of its engines somewhat. The benefit will be very slight before you get up to a sizable fraction of the speed of light, though, and it may not be worth the added cost of a giant and temperamental scoop on the front of your starship.
 * 2) The "fuel scoop". The best source of deuterium is a gas giant or a star, so some spacecraft in Science Fiction have the ability to scoop fuel from one of these two objects, even if they lack the ability to scoop up the (much thinner) interstellar medium. Sure, it's risky to dive through a Jupiter or a Sun in this manner, but it's cheaper than paying at the pump.
 * 3) The "ram brake," or magnetic sail. Conventional rockets not only need to spend fuel to speed up, they need to spend fuel to slow down. Even if it's impossible to scoop up the interstellar medium without inducing drag, you can still use that drag to your advantage when it comes time to slow down at the end of your journey. And if you don't have to carry along fuel for decelerating, you can make your space ship much, much smaller and lighter, even if your fuel supply still limits your maximum coasting speed.

Literature

 * Appears a lot in Larry Niven novels.
 * In Protector, the Pak use ramscoops to cross the 30,000 light-years from their homeworld to Earth. Eventually, after becomes a Protector, he ends up in a dogfight with another ramscoop, piloted by Pak who's out to kill him.
 * In Ring World, the eponymous ringworld has several ramscoops docked with it, left over from an earlier age when it received cargo from other star systems.
 * Poul Anderson's Tau Zero.
 * Vernor Vinge's A Fire Upon the Deep mentions ramscoops, particularly for use in the Lower Beyond. A Deepness in the Sky, set in the Slow Zone, features ramscoops exclusively. They use force fields to make big enough ramscoops.
 * In a novel of The History of the Galaxy series, the Alpha, the first extrasolar colony ship, is supposed to be accelerated by three extremely-powerful fusion engines (Captain's Log mentions that their combined power rivals a star). When the ship ends up in the Orion Nebula by accidentally tearing a hole between dimensions with the engines, they expend most of their on-board hydrogen tearing another hole back to our universe. The system in the nebula has no habitable planets, so the crew and the colonists are forced to live on one of the barren worlds for several generations, while the ship makes a long orbit through the dense part of the nebula, collecting hydrogen with its scoop. When the ship is found centuries later, it has only managed to replenish about a third of its stores. Not that it matters, as everybody uses Faster Than Light Travel by that point.
 * Hydrogen fusion ramscoops are mentioned, but never shown, in John Varley's Gaea Trilogy.
 * Ramscoops also get a brief mention in Gregory Benford's The Stars in Shroud, although FTL jump-drive technology is the main means of interstellar travel in that universe.
 * In The Pentagon War, interstellar "scramjets" were used before the advent of linked hyper holes. An experimental RAIR design that uses Antimatter appears in chapter 8.
 * Alastair Reynolds' novel, The Prefect, shows a mothballed Ramscoop powered interstellar ship, designed to allow interstellar travel without the Conjoiner's conjoiner drive. The ramscoop's massive magnetic fields are used to horrific effect as a weapon.

Live Action TV

 * Destiny in Stargate Universe can refuel itself by taking a brief dip into a star. Naturally, it gave the crew quite a scare when the ship ran out of energy and promptly aerobraked in a gas giant's atmosphere to head for the nearest star, resulting in them discovering just how it could remain operational for millions of years.
 * Exist in Star Trek. The red glowy things on the fronts of the Enterprise's warp nacelles are called "Bussard collectors", and were mentioned once a season.
 * Andromeda has one case of a ship collecting fuel from a recently supernovaed star.
 * Red Dwarf has a full scoop on the front of the ship.
 * Carl Sagan mentions the Bussard ramship design in Cosmos. His take on it was somewhat optimistic.

Video Games

 * Ramscoops and Ramjets are some of the equipment available in the 4X game, Star Ruler. As the ship moves, they slowly restore fuel, allowing you to have effectively infinite fuel, so long as you use a small rocket. Ramjets are basically a rocket and ramscoop combined, eliminating the need for fuel tanks.
 * Elite and its sequel has this as a purchasable option for your ship. Of course, you could still get attacked by pirates while heading for the star.
 * In Alien Legacy, all the colony ships sent from Earth use fusion-powered drives. The intro even shows a giant net unfurling from the Callypso in order to collect enough hydrogen to slow down in order to enter Gaea's orbit.
 * In Sword of the Stars, the Hivers, having no FTL capability except a Portal Network, can research Ramscoop technology that allows their ships to travel without expending fuel. However, unlike a "standard" Bussard collector, Hiver ramscoops are powerful magnets that do the same job.
 * Stars! have scooping engines -- when flying slowly, they produce surplus fuel, but have lesser optimal (and combat) warp speed than normal engines of the same Tech Levels and when exceeding it eat even more than normal ones. There's also "No Ram Scoop Engines" trait -- unless paired with Improved Fuel Efficiency, this means dependence on big starbases and fragile fuel plants. Radiating Hydro-Ram Scoop, the lowest commonly available on Tech Tree, gradually kills transported colonists in the same fleet unless the race got rather high radiation optimum (not even maximum) value. This means not only ships with rad-scoop are limited to hauling minerals, but e.g. armed escorts would have to dance around a transport or colonizer as a separate fleet without merging. Ships with scoops also suffer 1/4 more damage per engine from Space Mines.