Ringworld Planet

One of the World Shapes more often found in Science Fiction than in Fantasy, a Ring World Planet is a world that is a world in the shape of a concave cylinder. The horizon curves up, not down, but only in one dimension. The sides of the cylinder will be walls, with or without a "ceiling." These can range in size from a true Dyson Sphere to a cylindrical space station. These variants of worlds usually at least pay some lip service to the known laws of physics, since a spinning ring generates a centrifugal force that could be used instead of gravity. However to exist for real, particularly large ones would have to be made of Unobtainium.

Note that it would always be "day" in such a cylindrical world unless measures are taken to simulate day and night, either through sun shades, mirrors, or some combination of the above.

These were formerly referred to as "Niven's Rings" by physicists, astronomers, and science fiction writers, after the creator of the concept, author Larry Niven (who thought it up as a mid-point between a Dyson Sphere and a planet), in his novel Ring World.

Anime and Manga

 * The Gundam franchise helped popularize the O'Neill Cylinder space colony (see below in "Literature"), as well as other designs.
 * Actual ring shaped colonies (Known as the "Stanford Torus" or "Island 2" model) are only common in the Gundam Wing coninuity, though one also shows up in Gundam Unicorn, which was apparently the first ever built in the UC-verse.

Comic Books

 * A Defenders storyline centered on the Tunnelworld.

Film

 * You see one for a little bit in Revenge of the Sith.
 * Possibly the most famous example, the space station from 2001: A Space Odyssey.

Literature

 * Larry Niven's Ring World is set on a world shaped like a vast ring with a sun at its centre. It's made of Unobtanium called scrith and is so massive that its geographical features include 1:1-scale maps of several planets (including Earth). These maps are significantly less than 1% of the ring's surface area.
 * The Culture of Iain M. Banks's novels builds Ringworld-style Orbitals (but smaller) as housing for many of its citizens. They have a few full size, fits-round-a-star Ringworlds too but they're much rarer, since you can get more useable area by using the same mass to build orbitals so most of the Culture regards them as tacky.
 * Arthur C. Clarke's Rama from the series started by Rendezvous With Rama, is a massive cylindrical spacecraft.
 * The protagonists of Gregory Benford's Beyond Infinity spend a brief time trapped in a Tunnelworld after an encounter with some 4-dimensional aliens. It was a closed loop, so traveling in any direction for a long enough time would return you to your point of origin.
 * Gerard O'Neill proposed a real world cylindrical space colony: Island Three.
 * The page illustration is a representation of the "Stanford Torus", another design inspired by both O'Neill's work and the classic "wheel-and-hub" space stations.
 * Earth in Illium and Olympos is surrounded by two huge ever moving rings. They are not fun places.
 * Thistledown, from Greg Bear's Way Series, is a hollowed out asteroid containing seven cylindrical chambers separated by bulkheads.
 * The seventh chamber
 * The Anne McCaffrey novel The City Who Fought takes place entirely on a cylindrical space station.

Tabletop Games

 * Sigil from Dungeons and Dragons Planescape setting is this. It's also a sort of hub that connects to all the other planes of existence.
 * There's also Penumbra, the illithid homeworld, which is a full Ringworld. It may or may not exist yet.

Video Games

 * Halo takes place on a world (Installation 04) resembling Banks' Orbitals at the midpoint between a gas giant and its moon. All of the other Halos appear to orbit gas giants as well.
 * Startopia has you turn one of these into a profitable space station.
 * Several, actually. Apparently, all known races use the same exact design for their space stations, right down to the color scheme.
 * EV Nova has several of these, mostly ring-around-a-planet style. Though one is (for all intents and purposed) THE Ring World. (The Polaris use that one for effectively infinite farmland.)
 * In Mass Effect, the Citadel is one of these. In particular, the Presidium is a huge ring with its own biosphere and simulated sky at one end of the space station.
 * The Alliance's Arcturus Station is described as a Stanford Torus.
 * You can make these in the Space Empires series. A Dyson Sphere is better, though.
 * Some of the planets in both Super Mario Galaxy and Super Mario Galaxy 2, such as the planet in which you fight Megahammer (a Humongous Mecha piloted by Bowser Jr.) on in the latter, actually look like these.
 * You can build these in Star Ruler, admittedly as a lategame option. They are Capital-H Huge, larger than some planetary orbits.
 * Shores of Hazeron has ancient ringworlds which can be colonized. The ringworlds are almost exactly like those from the Ring World novel, with mountains flanking the inner walls, and with shadow squares creating day/night cycles on the surface.

Web Original

 * The world of Pendor, from The Journal Entries of Kennet Shardik, is Niven's Ring World with the Serial Numbers Filed Off, because Niven had threatened to sue Elf Sternberg over writing gay BDSM Known Space fanfic.
 * Darwins Soldiers: Card of Ten takes place primarily on one of these.