Counter-Earth

A hypothetical planet on the other side of the sun from the Earth which is like the Earth, but hidden. This idea has been discredited because its gravity would affect the motion of other planets, and other planets' gravity would cause it to drift out of its position. The idea still was popular in old science fiction, and keeps appearing in sci-fi which is not hard enough (which includes almost anything that isn't pure written science fiction).

The concept originated with the ancient Greeks and was known as Antichthon, which is Greek for Counter-Earth.

The Other Wiki has a list.

Not related to blocking the ground.

Comic Books

 * Several at Marvel Comics.
 * A Counter-Earth created by the Earth scientist, The High Evolutionary used to be there. He wanted it to be better than the real Earth, but one of his earlier creations, the Man-Beast, corrupted it. The evolutionary almost destroyed it in his disappointment, but the space hero Adam Warlock asked for the chance to save it, and was granted it. (If that sounds like a Christ analogy, it's because it was). The planet was eventually removed from the Solar system by Sufficiently Advanced Aliens and placed in a museum.
 * The Spider Man Unlimited TV series and the associated comic book took place on that one. The comic was not in-continuity with the rest of Marvel's comics however.
 * One was briefly created in Infinity Crusade: Paradise Omega, created by The Goddess (Adam Warlock's "good" side.)
 * The Earth of The New Universe was placed here.
 * Most recently the Heroes Reborn Earth was put in this position.
 * Recently, Superman's New Krypton positioned itself directly opposite Earth.
 * In the first few appearances of Superman, Krypton itself was in fact this. Presumably the writers believed even Krypton-level spaceflight technology would only be able to get that far...
 * Judge Dredd: Hestia is a planet which orbits the Sun at nearly the same distance as the Earth but at such an angle to the ecliptic plane that it was not discovered until 2009. It is inhabited by a small colony of humans and an intelligent indigenous population who keep their distance from the colonists. The planet is also home of the lethal Dune Sharks (flying shark-like predators which can burrow beneath the ground).
 * Les Cités obscures, a graphic novel series by François Schuiten and Benoît Peeters: stories set in a group of city-states on a counter-Earth.

Film

 * Doppelgänger (also known as Journey to the Far Side of the Sun in America): Astronauts discover such a world that not only looks like Earth, but everything that happens on Earth also happens there, like mirror images. This includes the astronauts' landing!
 * If everything that happens there is a mirror image, this doesn't explain why its orbital trajectory isn't!
 * The MST3K movie Stranded in Space a.k.a. The Stranger (the failed pilot for a TV series based on Doppelgänger) featured this premise; an astronaut crash-lands on the counter-earth, the government of which tries to study him and prevent him from learning this. After figuring out that he's not on the planet Earth he left (despite the similarity of technology, architecture, human physiology and the general aesthetic of the world at large) he spends the rest of the film trying to sneak onto a space shuttle so he can commandeer it and return home. He doesn't make it and the premise of the show would have been his adventures Walking the Counter-Earth as he tried to get home.
 * Gamera Versus Guiron had this as a plot device. The inhabitants were not friendly. Cue Gamera destroying stuff.
 * In Warning From Space (a.k.a. Mysterious Satellite), "Planet 'R'" is on a collision course with Earth. One-eyed, starfish-shaped aliens from the counter-Earth Paira take on human forms to warn the earth about the impending disaster.
 * Another Earth
 * In Melancholia, the counter-earth planet Melancholia in on collision course with Earth, but is first believed to be simply passing by.

Literature

 * Gor; the name of the series is in fact Chronicles of Counter-Earth.
 * To give Norman some credit, Cabot immediately points out that astronomers would be aware of it because of the gravitational effects and that it couldn't stay in position without a way to move it (it turns out the Priest-Kings take care of all that). This doesn't make up for Gor managing to share an orbit with Earth but still be closer to the Sun.
 * The Korad trilogy by F. Mond.
 * Planetoid 127 by Edgar Wallace is a short novel of communication by radio with another world on the other side of the sun in Earth's orbit.
 * The Antigeos series of novels including The Other Side of the Sun, The Other Half of the Planet) and Down to Earth by Paul Capon (also serialised on radio by the BBC) are set on the counter-Earth Antigeos.
 * La Dixième Planète by C. H. Badet.
 * La Planète ignorée by René Guillot.
 * Aïo, terre invisible by Christian Grenier.
 * In The Illuminatus Trilogy, the leaders of the Illuminati may have originated on a counter-Earth named Vulcan and come to Earth on flying saucers from Mars via Saturn.
 * The X12 series of books by Olof Möller prominently features a counter-Earth called Anti-Tellus.
 * Zillikian is a counter-Earth featured in the Bunduki series by JT Edson.

Live Action TV

 * In Lexx, a pair of planets orbit both each other impossibly close and our sun on the opposite side as Earth. This is actually part of the reveal at the end of the third season; these planets are
 * Space: 1999 used a variation on this one. The planets Beta and Delta were technically at war, but being on opposite sides of their sun they couldn't actually shoot at each other. Until the itinerant moon wandered along, that is, and the Betans and Deltans started using it, much to the annoyance of the Moonbase Alpha crew, as a missile base from which to attack each other. Which doesn't explain how or why the two planets were at war in the first place....
 * Or if they'd ever heard of non-line-of-sight weapons, or orbital mechanics for that matter... (answer: probably not).
 * The Cybermen in Doctor Who came from Mondas, a double of Earth. It was not on the other side of the sun in the episode. It originally was in the same vicinity as Earth, until something happened, shoving Mondas out of its gravitational pull and hurtling it out towards the outskirts of the solar system (what today we would call the Kuiper Belt). The people there cyberized themselves to survive, and then tried to move their planet back into place in the solar system, which is where we see them in their first appearance.
 * In a Saturday Night Live sketch, a character mentions, apropos of nothing, his firm belief in the Counter-Earth.
 * The character was Father Guido Sarducci, and he described the planet as nearly entirely Earth-like down to the civilizations; amongst the few differences described was the practice of eating corn on the cob vertically, instead of horizontally.

Newspaper Comics

 * The newspaper strip Twin Earths was built around this concept. The counter-Earth Terra orbits opposite Earth. The daily strip featured Vana, a Terran spy living on Earth to keep tabs on our technology, and Garry Verth, an FBI agent. In the Sunday strip, a young Texan named Punch explored Terra with its young prince Torro. This strip mostly consisted of travelogue-like views of Terran life, for example the fact that in their liberated society, women, who constituted 92% of the population, ran things.

Radio

 * On The Adventures of Superman radio series, the planet Krypton is said to be "situated on the other side of the Sun" from the Earth in the first episode.
 * The 2000 Plus episode “Worlds Apart” involves a planet “exactly opposite the Earth, on the other side of the sun” (but, inexplicably, slightly closer) named Vesta.

Tabletop Games

 * Mage: The Ascension: a planetoid called Autochthonia exists in the Counter-earth position in the game's cosmology. This is the location of The Computer which is central to Iteration X, the cybernetic convention of mages.

Theatre

 * Sera Myu: A planet called Vulcan is said to be on the other side of the sun.

Webcomics

 * It is suggested in Eight Bit Theater that the planet which the story takes place on was in the same orbit as a different planet on the other side of the sun, where giant lizards were the most prominent life-form. Evidently, it is Earth in the Mesozoic Era. Here is the comic in question.
 * Fansadox issue Orc Counterpart Earth, a parallel universe that can be entered into through either science, magic or a combination of the two.
 * Tom the Dancing Bug has a Counter-Earth that is "not quite the opposite of our own...but somewhat dissimilar in certain ways!"

Western Animation

 * In Dinosaucers, the title characters and their enemies the Tyrannos came from a counter-Earth called Reptilon.
 * In Sport Billy, the eponymous hero is from the counter-Earth Olympus, populated by athletic god-like beings.

Real Life

 * Believe it or not, there actually is a way that this trope could be justified. The counter-Earth "point" (actually a circular orbit) is L3, one of the five Lagrange Points (actually closed orbits in a three-body system). Unfortunately L3 is not a stable orbit: if the body is even a millimetre out of position, or if it is perturbed in the slightest (say by the gravitation of Venus, Mars, Jupiter) it will move into a chaotic trajectory and eventually leave the vicinity of the L3 point in an unpredictable fashion. This would take on the order of only tens to hundreds of thousand years (less for large perturbations), which is a geologically short time.
 * There are five Lagrange Points. L1 and L2 lie directly in front of and directly opposite the Sun from Earth's perspective, at the points where the sun's gravity and Earth's add or cancel just the right amount to produce a circular orbit with the same period as Earth's year. A counter-earth would be at Earth's L3 point. However, L1, L2 and L3 are unstable; if an object was even one millimeter out of alignment (easily happens due to there being other planets around, perturbing orbits and mischief like that) the forces would be out of balance and the object would slowly drift away from the Lagrange point. L4 and L5 are 60 degrees ahead of and behind the Earth, and are also called the Trojan Points after the Trojan asteroids, which lie around Jupiter's L4 and L5 points (any time a light object orbits a heavy one, there will be Lagrange points). The L4 and L5 points are stable (a small perturbation would put the object into a dynamically-stable, quasi-periodic Lissajous orbit that would keep it near the Lagrange point), but any object there would be easy to see: an object in the L4 point would be a perpetual morning star, rising four hours before the Sun every day and preceding the Sun on its annual circuit of the ecliptic, and an object in L5 would be a perpetual evening star setting four hours after the Sun every night, and trailing the Sun around the ecliptic.
 * The fact that the Lagrange point model assumes there are no other objects of comparable mass to the planet in the same orbit means it cannot be relied on to make a conclusion one way or another about how a three*-body system would behave. That said, such a system would not be stable. *The Earth-Moon system counts as one body.
 * There is another possibility, called a horseshoe orbit. A horseshoe orbit does not require vast differences in mass, and an object in one with Earth could temporarily be opposite the sun. However, it would still be detectable.
 * A hypothetical body in that formed in the L4/L5 spot may have created the Moon.