The Multiverse



Sometimes a writer creates a universe almost exactly the same as ours, but with slight differences. Some settings refer to not just one other dimension, but to a whole set of other dimensions and universes. A system of distinct worlds exists, often interconnected in a way that allows characters to travel to and from them. It might be as a tourist who just goes to look and tries not to change anything, or as a participant who goes in and interacts with the people in the other universe.

Basically, the travelers have cross-universe travel powers. While it is possible they don't have a choice in their destination, sometimes they do. Most often the means of traveling are special gates, either naturally occurring or by technological or by other magical Phlebotinum.

In general, there are two types of this Trope, though they may overlap:

Many Worlds: The main setting is one of many dimensions that are entirely different and unique, such as Oz, Narnia, Wonderland, that sort of thing.

Branching Timelines: The main setting is one of many dimensions that are similar overall, but have differences that set them apart. For example, characters who are Good in the main setting are Evil in one of the alternate worlds (or vice versa), technology or cultural level is different in one of them, historic events happened differently, etc. New timelines are formed by (significant) decision points in existing timelines, with a new universe forming for each possible alternative. In some versions, branching timelines will sometimes merge back into the original timeline when their differences turn out to be insignificant or "damp down" after a while.

Regardless of the variant, the multiple worlds of a Multiverse are usually "contained" in something far vaster than them, and usually having more dimensions ; if this is a "space" of sorts that can be traversed to move from universe to universe, then you have the Void Between the Worlds.

In some occasions it's the way for a creator to tie several different works via Canon Welding, or to create Crossover.

This provides all sorts of interesting ideas for things you can do, for good or bad. If it involves trans-universe Sex Tourism, you have "Your Universe or Mine?" (or Screw Yourself sometimes). However, beware Evil Twin, and similar beings.

Sometimes, the Multiverse is protected by a Guardian of the Multiverse. The Multiversal Conqueror, on the other hand, wants to conquer or destroy it.

Not the same thing as Alternate Continuity, but it makes a handy way to link them if the writers are so inclined. See also Bizarro Universe, Another Dimension and Alternate Universe. Compare Rubber Band History.

For works set in the same universe, see The Verse.

Anime and Manga

 * Yuuko from ×××HOLiC is called the Witch of Dimension for her ability to create gates to any Alternate Universe. It's her services which allow the cast of Tsubasa Reservoir Chronicle to hop for one dimensions to another trying to collect all the feathers.
 * Noein has this where every universe is a For Want of a Nail situation, running off the theory that for every decision made, Another Dimension is created where the other option was chosen. Much philosophy ensues.
 * While on the surface Digimon appears to be a case of regular continuity reboots, the Wonder Swan video games establish it instead as a multiverse. Almost every anime series, manga and video game of the franchise has its own entirely separate versions of the real and Digital World (except Digimon Adventure 02 and Digimon Xros Wars: The Young Hunters Leaping Through Time, which are sequels and so exist in the same world); the Wonder Swan games follow Ryo Akiyama's travels between a few of them. Digimon V-Tamer 01 had three special chapters in which Taichi Yagami and Zero visited other Digimon universes, namely those of Digimon Adventure (which has its own version of Taichi Yagami), Digimon Frontier and the Wonder Swan games. As such, almost all incarnations of Digimon take place in a different human world/Digital World pair in the multiverse, and if you know what you're doing (or have a dimension-jumping nemesis, in the case of Ryo Akiyama) you can travel between them. In addition, some canons have other dimensions tied to that particular universe; Digimon Adventure had the Dark Ocean and a surreal world wherein one's wishes defined reality, with others accessible through card combinations at a gate in Vamdemon's castle, and Digimon Xros Wars: The Young Hunters Leaping Through Time features DigiQuartz, a bizarre Dark World in between the Real and Digital Worlds.
 * The setting of Magical Girl Lyrical Nanoha is advanced enough to have fairly casual inter-dimensional travel, with the Time-Space Administration Bureau monitoring the security, safety, and cultural growth of every dimension.
 * Tenchi Muyo! is the one huge metaseries with literally tens of alternative continuities, explored in various TV series, OVAs, novels, mangas, doujinshis etc. It also includes other series and titles by the same author and studio, and even the works by the other creators with sometimes completely different premises. It was explicitly stated by its creator, Masaki Kajishima, that all these titles and interpretations are true and represent different aspects of the same Multiverse, and he doesn't mind sharing. On the other hand, he restricts himself to the Tenchi Muyo! Ryo-Oh-Ki (that is, original OVAs) storyline and its spinoffs like Tenchi Muyo! GXP, and consider it the main and central storyline.
 * Further, Dual! Parallel Trouble Adventure is known to be another universe in the Tenchi multiverse, and there are hints that El-Hazard: The Magnificent World may be as well.
 * Bokurano: The main characters are fighting aliens who are attempting to destroy the Earth. Eventually it is revealed that.
 * Though not confirmed officially, the Evaverse is implied to use this trope between the various continuities and spinoffs. Kaworu appears to be aware of and/or remember every single one, including the anime, the mangas, the games, even the Rebuild films. If true, it means he's the fucking creepiest Stalker with a Crush and Memetic Molester ever since he's stalking his (male) love across several universes.
 * The Slayers takes place in one (sometimes called “the red world”) of four universes, each of which has a good and evil god with five subordinates a piece, all projections floating atop a multiversal golden sea of chaos, A.K.A. . This doesn't really come into play for the most part, aside from in the anime-original TRY season, which concerns “overworlders” searching for five Plot Coupon weapons from their universe…
 * Though the author says it's not so, Fanon holds that Lost Universe takes place in the “black world” from Slayers TRY.
 * When They Cry uses and abuses this concept for its Groundhog Day Loops. The multiverse can be viewed only in the sea of kakera/pieces/fragments. In that sea, each fragment is essentially one possible path that the world can take. Umineko no Naku Koro ni introduces the idea of voyager witches, such as Bernkastel and Lambdadelta, who are the only beings capable of traveling the different fragments, and creator witches, who are the only ones capable of creating new fragments.
 * Katekyo Hitman Reborn has  become a Physical God of all parallel universes, using the three sets of rings and pacifiers in order to do so.

Comic Books

 * The comic Fables mainly features our own world, but also a indefinite number of Fairy Tale worlds which are connected by gates. The gates to our world are closely guarded by the New York Fables since they were conquered by the Adversary.
 * This latter plays a role in the War against the Adversary.
 * Major staple of The DCU and Marvel Universe comics (although DC attempted to get rid of it in the '80s; it's back now).
 * The Wildstorm multiverse consists of a 196833-dimensional snowflake-shaped arrangement of universes, flat sheets of information that their inhabitants interpret as three-dimensional (or what have you). Between them is the Bleed, red swirly stuff of vaguely-defined properties. The Bleed, if not the snowflake, has recently been incorporated into The DCU.
 * After the reboot/relaunch in 2011, there is no "Wildstorm multiverse" anymore, and all the Wildstorm characters and stories are incoporated into the new "rebooted" DC Universe. And then DC goes and (re)introduces Earth-2 in its second wave of releases after the relaunch.
 * The DCU and Marvel Universe are also part of one multiverse as evidenced by crossovers that have been referenced later by both companies (making them canon). Both also contain multiple alternate futures and the characters from the past and future of other universes. Now, go back and read that again.
 * The DCU multiverse has now been pared down to "just" 52 universes. Plus characters who travel through time of course.
 * In fact, Marvel (or perhaps it was DC) introduced the concept of the Omniverse, essentially a multiverse of multiverses.
 * Warren Ellis' run on X-Man utilized another conception of the multiverse, where in addition to Parallel Universes, there's a "spiral of realities" stretching above and below, with the universes "downspiral" being significantly more chaotic and difficult for live to develop/survive in than the the relatively advanced and idyllic universes located "upspiral".
 * There are in fact two types of alternate universe in the Marvel Universe. The first are different dimensions adjacent to the regular 616 verse, such as the Microverse and the various magical verses; the second are parallel universes, which are usually alternate timelines, and presumably each has its own adjacent verses too. It's not always clear which is which- the Negative Zone, for instance, is vague on if it is the first or second kind, so the one encountered by the regular Fantastic Four and their Ultimate counterparts could actually be one and the same (and the Nihil encountered by the latter wasn't a counterpart of the regular FF foe Annihlus, but a member of the same species- which is why he referred to himself as a member of a "caste" and already knew who Reed Richards was in their first meeting).
 * The comic books Those Annoying Post Brothers and Savage Henry focus on Bugtown, a infinite-sized City of Adventure which connects to an infinite number of alternate worlds.
 * So did the comic book Grimjack.
 * Zenith, a 2000 AD series, features this. Perhaps not surprisingly, its author Grant Morrison is one of the creators responsible for bringing back the DC multiverse.
 * The Archie Sonic the Hedgehog comic features multiple universes, but calls them 'Zones'. This includes: The 'Prime' Universe, a Mirror Universe, the Sonic Rush Dimension, the Sonic Underground Universe, the Special Zone, the No Zone, the Interdimensionsonal Highway (a zone between zones), and various others. Including a Sailor Moon-inspired universe!
 * Oddly, there has been no mention of the original SEGA universe, beyond some ads for new games.

Fan Works

 * The Multiverse plays an important part in Super Milestone Wars and its sequel.
 * The Open Door is centered around the concept of the so-called heroes exploring the Mega Crossover multiverse in search of ways to improve themselves in preparation to Punch Out their enemies. Universes are sorted according to an "energy gradient", which roughly serves as a Sorting Algorithm of Evil by paralleling each universe's capacity for technology, magic and psychic power. Parts of it have been sealed off and can only be accessed from other parts of the multiverse through "hub universes", with that of Suzumiya Haruhi serving as one of these. Interestingly, it also has Alternate Universes; the second chapter shows that at least one version of Neon Genesis Evangelion different from the native Thousand Shinji one exists, while later chapters also show that  also exists.
 * JLA Watchtower and DC Nation laughed off "52 Pick Up" (as referenced above), going with the older concept of Hypertime.
 * The Chance Encounter crossover series uses this as a plot device, with the main characters hopping around a collection of universes (LOTR verse, Kingdom of Heaven verse, POTC universe and the Troy universe), mostly by means of shipwreck.
 * The travel method has become something of a Running Gag, with one of the main characters (Balian) being regularly advised to stay on land. Even that doesn't work. Mostly they tend to arrive in tree's, leading to Legolas remarking that whoever organises these jumps has a serious lack of imagination.
 * With Strings Attached takes place in or mentions at least six different universes, and Jeft refers to existence as the Infiniverse.
 * Stars Above uses this concept.
 * Drunkard's Walk is a Mega Crossover that is set on more than a dozen different Earths, one at a time. Based on a message from someone who should know seen at the end of Drunkard's Walk II, it appears to be a mix of both types listed in the main text, with many worlds that each may spawn their own branching (and re-merging) alternates.
 * A Brane of Extraordinary Women documents events occurring in several universes within a Multiverse, one of which is The Teraverse.
 * The late 1990s fic cycle Twisted Path is probably the Trope Codifier at least for anime fanfiction, and inspired numerous other fic projects, some of which have had their own explicit crossovers with it.
 * The universe of Undocumented Features is part of a mulitverse (which includes, but is not limited to, at least one Star Trek universe as well as the world of Marvel Comics), but outside of a few special storylines very little is seen of these alternate realities.
 * My Apartment Manager is not an Isekai Character is set in a "Many Worlds" multiverse which is undergoing a massive crisis which results in characters from numerous other worlds finding themselves displaced to a single common "low-energy" world, where they are put up in residences established for that purpose by the forces of Heaven and Hell, who are working together to keep the multiverse from unraveling.

Film

 * The basic premise of The One is that Gabriel Yulaw defected from a group known as the Multiverse Authority after killing one of his counterparts. By killing his counterparts he is able to absorb their power and become more powerful. His new goal in life to destroy every single one of his counterparts and become a God, not knowing that his last counterpart is equally as powerful.
 * The Big Bad in the movie Last Action Hero discovers since he can cross over to other movies and other worlds, he can bring back the worst of the worst villains. They'll have a formal party: Freddy Krueger, and Jason can supply the meat, Hannibal Lecter can do the catering, etc. And it will all take place in what is apparently the "Real World", because here the bad guys can win!.

Literature

 * The trope's name comes from Moorcock's books. His many books range a vast array of worlds yet a sizable proportion of them are connected via Canon Welding. Robert A. Heinlein used this name in The Number of the Beast (see below) as well.
 * Roger Zelazny's Book of Amber has one reality, Amber, which casts an infinite number of Shadows, each one a full world (with Earth among them) or some strange reality area (like that bar where Cheshire Cat hangs out, or bridge-linked islands floating in a starry sky without any ground below). The Princes of Amber can travel at will to these worlds by using Tarot cards as portals (some can make their own, but it's uncommon), or by walking the shadows and altering them until they stand in the world they desire. On the "opposite end" are Courts of Chaos — the Rim, and beyond it there are no Shadows, just Chaos. Then there's "land between the shadows" aka "negative space". Then there's the "mirrorworld".
 * Diana Wynne Jones's series, stand-alones and short stories often feature Multiverses or at least two different alternate realities. Count them:
 * The Chrestomanci Chronicles
 * The Dark Lord of Derkholm
 * Howl's Moving Castle
 * The Magids series
 * The Homeward Bounders
 * A Sudden Wild Magic
 * The Game
 * Everard's Ride (this one's a novella)
 * "Dragon Reserve, Home Eight" (this is a short story)
 * One might theoretically consider A Tale of Time City as well. Time City is not technically another world per se, but it runs along the much of the same ideas.
 * His Dark Materials features many an Alternate Universe, with a few powerful items allowing one to travel between them.
 * Robert A. Heinlein has written about Multiverses more than once. The novel Glory Road has magical inter-universe travel, and several of his later novels (starting with The Number of the Beast) involve the Burroughs drive, an invention that lets you travel to other universes; but because of how the Multiverse works, those nearby universes will be the favorite fictions of the users of the drive. So you can leave Earth and the next universe over will turn out to be Oz... As a side effect, Heinlein managed to tie together practically everything he ever wrote into a single setting.
 * Variation: Timothy Zahn's Cascade Point stories feature a faster than light drive system which has the side effect of showing you alternate versions of yourself whenever you activate it, based on different possible outcomes of your life. At least one story features a Phlebotinum Breakdown that drops the ship into one of those universes. It is a Multiverse, but it's not one where you can easily travel between worlds.
 * The Riftwar Cycle, by Raymond E. Feist involved several universes with magical travel between them. It's always dangerous though, because opening a hole in your universe may attract the attention of beings who were (barely) defeated once by gods...
 * The Narnia books mainly feature travel to and from the titular Narnia, but in The Magician's Nephew it's explained that our world and Narnia are only two of a Multiverse of worlds. We only ever see three, though. Four, if you count.
 * 4.5 really, with the Wood between the Worlds, which is different from the others like a line is from a square
 * Word of God says the Woods between the Worlds was a part of Aslan's country. Which makes it kinda creepy when you realize that according to The Magician's Nephew
 * The Discworld novels often allude to a multiverse. Since all libraries, everywhere, in every space, universe and time are connected, you can reach this L-space in the Library of Unseen University. If the Librarian lets you in, of course.
 * Also, in The Colour of Magic, Rincewind and Twoflower briefly find themselves occupying/incarnated in/deluded into thinking they are alternate universe versions of themselves, sitting next to each other on an airplane in a world that appears to be our normal Earth.
 * Our Earth plays a greater role in the Science of Discworld series, where the wizards know it as "Roundworld".
 * Moreover, the existence of alternate worlds (i.e. different legs of the Trousers of Time) is a given in several Discworld novels. Granny Weatherwax starts picking up random memories from these alternate worlds in Lords and Ladies, and Sam Vimes accidentally swaps P.D.A.s with his  counterpart from an alternate world in Jingo.
 * Also in Lords and Ladies Ponder Stibbons (a wizard version of a physicist) tries to explain the "many worlds'' principle to the Archchancellor (chief Wizard). Of course this goes badly as the Archchancellor is mainly concerned about why his trousers have anything to do with it, and why his parallel self never sent him a wedding invite...
 * Introduced as the Whole Sort of General Mish-Mash in the fifth part of the The Hitchhiker's Guide to the Galaxy trilogy.
 * In The Wheel of Time, there is a multitude of possible other 'verses, called the 'Worlds That Might Be', which are basically Alternate Universes. The less likely they are, the more faded they look. A channeler can be transported to one of these by using a Portal Stone and the One Power.
 * Anathem by Neil Stephenson combines this with of all things Platonic Epistemology to very severely deconstruct many aspects of the multiverse. For example, atomic nuclei are subtly different between universes so any reaction between molecules from different universes is retarded (i.e. breathing).
 * Colin Kapp's The Dark Mind (also published as The Transfinite Man) uses this. The alternate universes were originally empty until mankind (in the form of the evil Failway Company) discovered them and, basically, turned them into vice dens.
 * Stephen King's Dark Tower series and the numerous books that feed into it (or got sucked into it) is based on the notion that all of the weird alternate realities visited by the Gunslinger, plus all of the alternate continuities of King's earlier books are part of a Multiverse that are all connected by the titular tower. All these worlds are apparently subservient to the world in which the fictionalized version of King lives, and the characters discover that they are all in fact being channeled by King the novelist, a la Stranger Than Fiction.
 * Star Trek has recently been indulging in a "Myriad Universes" series of books and comics which explores countless other realities throughout every Star Trek series, based on various differences in their timelines. Some of them vary hugely from the original while others vary in slightly simpler ways. There's also the original Mirror Universe.
 * One notable example of the Alternate Universe series is the Last Generation comic. Set in a universe ruled by the Klingon empire after a plot change during The Undiscovered Country (i.e. Kirk wasn't fast enough to save the president, ). In this Dystopian version of the Trekverse, Sulu captains the last remaining Star Fleet vessel as a Mysterious Protector, Tasha lives, Jean-Luc Picard leads a Resistance movement, Wesley took a few levels in badass (well, physical prowess, anyway) and Data is the key to changing history. Either that or he's the key to the most destructive computer weapons system ever created.
 * Kim Newman's novels are set in a multiverse, with different versions of the same characters showing up in Anno Dracula, The Diogenes Club, his Warhammer Fantasy Battle work etc. The Diogenes story "Cold Snap", which ties together several of his books, describes Keith Marion as being able to "shift" his mind between alternate histories (Marion's previous appearance had been in Life's Lottery a Choose Your Own Adventure-style book). The Cold, the Big Bad of the story, had previously appeared in Newman's Doctor Who novella Time And Relative, which makes the Whoniverse part of the Newman Multiverse.
 * Perry Rhodan is explicitly set in a multiverse, but deliberate travel between different universes isn't easy or necessarily safe. Problems involve the (at Galactic tech levels) largely unsolved issue of trans-universal navigation and the problem of adapting to the new physical laws of the destination, which has resulted in "strangeness shock" rendering entire starship crews comatose for months on arrival in the past. The focus of the action thus remains on the default 'main' universe, though interaction with others (including one in which time runs much more slowly, one that consists entirely of antimatter, and one that was and presumably still is heading for an artificially accelerated 'big crunch') has occurred in the past.
 * Kate in Choices by Deborah Lynn Jacobs. After her brother dies, she starts traveling between new universes with only slight changes from her own. For example, in one she was a pushover. In another she was a Perky Goth. She would usually go to sleep and wake up in a new universe.
 * The Coming of the Quantum Cats, while generally forgettable, has some fun with this trope. The great scientist Dominic Desota invents a machine for travelling between universes, but in one universe it's stolen by General Desota, who aims to become a Multiversal Conqueror. Opposing him are the heroes, Senator Desota and Nicky Desota.
 * The entire point of the Paratime series by H. Beam Piper was that the main character's culture had exhausted its resources and was sponging off the entire Multiverse.
 * Sergei Lukyanenko's Rough Draft-Final Draft duology is set in a multiverse where travel between universes is accomplished using the seemingly magical Towers.
 * Transition by Iain Banks
 * The Quantum Enchantment trilogy and its sequel by Kim Falconer uses the 'many-worlds' concept as a plot device. Working from memory, there is a dystopian Earth a few hundred years in the future, and a completely separate world where magic and the like are common place. Throughout the series, the characters manage to get themselves lost in the 'corridors' between the worlds, often returning to what would be their home but for some sort of twist- a battle was won instead of lost, time flows in a different direction, person X never existed. It gets somewhat confusing after a while.
 * Vasili Golovachov's The Saviors of the Fan duology (made up of The Envoy and The Deliverer) has a myriad parallel worlds some of which are similar to ours, while others vary wildly. And that's just those organized into a linked structure called the Worldfan, of which our world is a part, with the implication that there are countless other worlds. According to the protagonist's half-Japanese friend, some of these worlds may be familiar from folk tales or science fiction/fantasy novels, as information has a tendency to "leak" between the worlds, meaning all those stories are actually true in other worlds. There is a world where the mere act of moving alters the surrounding reality, or a world made up of a gigantic tree.
 * The Crystal-verse series by Vladislav Krapivin. "Great Crystal" being the model of Multiverse most commonly used by those interested in such matters. The variant worlds and some stranger places are mostly navigable via "facet intersections", where one can walk between the worlds, though usually are anisotropic, open periodically rather constantly, and accessibility of edge areas (thus set of routes available for a given individual) varies. Talents for using and finding routes vary wildly, and are trainable. Some people also can travel anywhere "directly", that is rip themselves out of the current continuum, to float in non-space for a few seconds and be dropped in another place, usually wherever desired, though their visualisation of process and attitudes vary wildly (on the average they find it very unsettling and avoid unless needed). And there are "mechanical" means, if less than convenient ones. Worlds and places range in Multiverse-awareness from not having such theories in common knowledge (you may meet "our world" there, unless it's that other one, or… etc), to having regular local fairs where people can buy unusual confections and cool weird trinkets without caring about their origin, to studying Multiverse in ways resembling radio astronomy, to treating services of a guide for interspatial travel as yet another legitimate business (if somewhat exotic one).

Live-Action TV
"Dick: You may see people you know too. But brace yourself -- they may have made different lifestyle choices. Harry: I see, in this universe Officer Don could be an old Vietnamese woman. Sally: No, they have the same bodies. Dick is saying they'll all probably be gay."
 * The TV show Sliders did this, although the heroes had no control over where they ended up each week.
 * Kirk, McCoy, and several others were transported to a Mirror Universe in the "Mirror, Mirror" episode of the original Star Trek, in which a dark Earth-based empire ruled the galaxy. This was very much an In Spite of a Nail universe, since everything was much the same except the moral/ethical bent of the Federation's counterpart and its citizens.
 * Years later, the cast of Star Trek: Deep Space Nine returned to this universe to discover that the revolution Kirk had encouraged its native Spock to foment had happened; unfortunately, its effects were not necessarily for the better.
 * Star Trek: Enterprise revisited this again in "In a Mirror, Darkly", just to hammer it home that Humans Are the Real Monsters.
 * Star Trek: The Next Generation had an episode with Worf bouncing between various In Spite of a Nail alternate universes. According to Word of God, this is distinct from the usual Mirror Universe though.
 * Charlie Jade: the Alphaverse (a Crapsack World and a Dystopia) is running out of clean water from its excessive pollution. So they use a portal to steal water from the utopian Gammaverse. Charlie Jade ends up in the world in between—the Betaverse, a.k.a. our world—when things go wrong with the portal. Later on he finds out that the halucinations he's had through his life are actually, and he returns home before deciding he liked the Betaverse better than Alphaverse. Long story short, the whole series is based around this trope, including the cliffhanger season finale (from which the show didn't get renewed) that would have revealed a fourth universe.
 * In Lexx there are only the Light Universe and the Dark Zone. And the Dream Zone. And the Other Zone.
 * In Kamen Rider Decade the titular Kamen Rider Decade must travel through nine Heisei-Era Rider Worlds to save the Multiverse...only to discover there are a lot more than nine. Decade also crosses over with Samurai Sentai Shinkenger, a world without Kamen Riders.
 * In the middle of the first season, Fringe suggests a multiverse through the ZFT Manifesto. In the season finale.
 * In 3rd Rock from the Sun there is a two part episode where the aliens visit an alternate universe (because they are bored) where they settled in New York instead of Ohio.


 * Stargate SG-1 had a handful of episodes where characters traveled to alternate timelines. The first season finale's storyline was kicked off when Daniel accidentally activated an alien artifact and went to a timeline where he had never joined the Stargate program. A later episode used the same artifact to help an Earth in an alternate timeline that was faced with looming invasion, and other ways of accessing alternate universes were later found as well.
 * Doctor Who is a bit tricky. There are alternate timeline-style universes, like the fascist universe of Inferno, or Pete's World from "Rise of the Cybermen"/"The Age of Steel"; post-the Last Great Time War, it's potentially multiverse-destabilising for a TARDIS to hop between such universes. There are also pocket dimensions linked to the main universe, such as E-Space or the Land of Fiction.

Tabletop Games

 * The Tabletop Games Rifts is part of Palladium Books' "Megaverse", which includes all of its other games (naturally), plus a number of other realms with some of the games.
 * The Tabletop Games Planescape, and by extension, the computer game Planescape: Torment.
 * And, through further extension, the whole of Dungeons & Dragons, though there are no or few crossovers between different game settings, such as Eberron showing up in the Forgotten Realms.
 * Spelljammer originally combined Greyhawk, Dragonlance, and Forgotten Realms, though like Planescape it can be extended to other settings that don't explicitly say their world isn't in a crystal sphere. (And of course, there's nothing that says Planescape and Spelljammer can't coexist.)
 * Planescape in fact explicitly includes the Spelljammer setting as part of its multiverse, along with all the aforementioned settings, as well as Birthright, Dark Sun, Ravenloft and implicitly every D&D setting ever devised, including contradictory ones.
 * Ravenloft is yet another setting that could bridge the divide between different D&D settings, including otherwise-inaccessible ones like Mystara, Athas, or Gothic Earth.
 * Also, Neverwinter Nights, set in the Forgotten Realms universe, explicitly stated that Bags of Holding were invented by "the kender of the far-off world of Krynn."
 * The long out-of-print Tabletop Games Torg dealt with the invasion of Earth by several different realities (cleverly typified as various classic roleplaying genres), each of which was trying to change our local physical laws to be like their own.
 * The trading card game Magic: The Gathering is set in a multiverse, with each expansion set representing a new universe (well, mostly). The game also implies that the players represent Planeswalkers, mages able to travel from one universe to the next essentially at will. The cards played represent magic and land discovered while exploring a given universe.
 * The casual variant Planechase literally represents the planeswalking with plane cards that affect gameplay and get cycled in and out as players shift to new worlds.
 * The GURPS supplement Infinite Worlds provides a setting and mechanisms for not only setting up a Multiverse incorporating all other GURPS books as a background against which to play, but also providing an interdimensional cold war as a driving force behind a potential campaign.
 * Wizards of the Coast long ago published a set of generic supplements for handling deities in roleplaying games, called The Primal Order. One of the books in this series, Chessboards, covered in exquisite detail how to design and manage an entire multiverse complete with cosmology.

Toys

 * Transformers has a whole pile of alternate universes which sometimes cross over, and which Hasbro and Takara disagree over which are actually separate and which simply occur to the side of other stories. The Transformers of the Axiom Nexus have grouped all continuities into a number of universal streams, with each stream corresponding to a continuity group. Thus, for example, Primax is the G1/Beast Era family, Tyran is the live-action movies, Gargent is the GoBots, Quadwal is the real world, etc. Some of these are negative-polarity universes in which Decepticons are good and Autobots are evil; these are assigned negative numbers. To make things really nuts, there are also multiversal singularities, of which only one exists in all reality. Some of these, such as Alpha Trion, exist in every universe simultaneously, while others, like Vector Prime and The Fallen, travel between universes. There are also sparks that resonate across the universe, giving rise to multiple similar but separate versions of Optimus Prime, Megatron, Starscream, and various others.
 * It should be noted that this is All There in the Manual. There's no sign in any of the televised series or available-at-your-comics-shop comic books - that is, any canonical stories - that you could "slide" from Transformers Armada into Transformers Animated, despite years of "collector's club" and convention exclusive materials that suggest you can. Fingers are still crossed around the fandom for a Turtles Forever sort of project someday, somehow.
 * The Transformers Aligned Universe, which includes Transformers Prime, the War for Cybertron video game series and the novels retelling the narrative of the games, are apparently considered to be outside the multiverse.

Video Games

 * The Myst games (and books) are primarily concerned with a magical means of inter-universe travel (and universe creation, if you believe Gehn).
 * In Myst III: Exile, Saavedro seems to be capable of annihilating the new D'ni age by destroying the age definition book, so Gehn has a definite point.
 * Later games and novels have explained that destroying the book only destroys the link to that particular world, although since said link can never be recreated, it might as well be considered destroyed as far as travelers are concerned.
 * Furthermore, though official disposal of Linking Books was historically handled with care due to issues of the special techniques involved in forging the world link, there's very little real harm in the action. Much greater harm comes from writing carelessly which can introduce unstable elements into the world described. The original Myst touches on this with Atrus's musings on the Stoneship Age and this is a major problem for the second game Riven.
 * The MMORPG City of Heroes has Portal Corp., a company dedicated to exploring other dimensions. There are several individual missions and arcs where you get to deal with hostile other-dimensional entities, either after they've entered Paragon or at their source. The current state of the City of Adventure can be traced back to a massive invasion from Another Dimension.
 * One such alternate dimension, Praetoria, is the focus of the latest expansion, Going Rogue, which fleshes out that dimension beyond Crapsack World Mirror Universe by adding moral complexity and considerably more shades of grey.
 * Super Paper Mario has this as the plot. Seeing as an Omnicidal Maniac is walking about with a dark book has the future written in it with the way how to end all of existence, Mario ends at a interdimensional crossroads to collect the Pure Hearts.
 * Makai Kingdom introduced the multiverse concept for the various netherworlds and non-netherworlds introduced by prior Nippon Ichi games (prime amongst them Disgaea).
 * This is major plot point in the Super Robot Wars Original Generation series with the revelation that Gilliam Yeagar is the same character as the main villain of Hero Senki: Project Olympus, and he has also crossed over into other dimensions to get back to Elpis. Dark Brain from Super Robot Wars Original Generation Gaiden is implied to be the same Dark Brain as the one from The Great Battle IV, as well as the creator of Dynamis from Super Robot Wars R.
 * Super Robot Wars Z has mysterious things call Sphere which allow its holder to travel and summon people between dimensions. The Edel Bernal even summon himselves (no, it isn't a mistype) to aid him in last battle.
 * The Nasuverse is actually made of different different continuities, possibly because the main works are multi-route visual novels. However, actually traversing between Universes is the domain of Magic (i.e. a miracle) so only one character can actually do it.
 * Also used as a source of infinite Mana by taking a small amount from an infinite number of alternate realities, using a tool designed by the aforementioned character. Incidentally, this is also a perpetual motion machine since it powers itself.
 * In Ratchet and Clank Future: Tools of Destruction, the all-knowing (for the most part), IRIS Supercomputer mentions that existence is divided into infinite dimensions.
 * The Nameless Mod mentions that other forum cities exist, and that travel between them is possible. Apparently a Planet Diablo merchant is responsible for the mana potions being in the game.
 * The multiverse is a key point to Suikoden Tierkreis, though the player/lead PC only technically ever gets to see one (the reason why is also a plot point).
 * Touhou Project involves quite a bit of world-hopping. Based upon a relatively Buddhist notion of the universe, like the Nippon Ichi example above, with multiple hells (Jigoku) and netherworlds (Makai), it gives the heroines plenty of reason to go To Hell and Back. This seems to have been more frequent earlier on in the series, when there weren't as many characters, and Gensokyo wasn't as fleshed out. Gensokyo itself seems to be a semi-self-contained world kept vaguely apart from the real/outside world by the Great Hakurei Barrier, in spite of being physically built in Japan. Alternate worlds the heroines may visit include:
 * In the original game, Highly Responsive To Prayers, Reimu has to close an open portal to Jigoku / Hell.
 * Phantasmagoria of Din.Dream features "scientists" who visit Gensokyo from an alternate dimension in order to "study danmaku" using a "probability hyperdrive vessel".
 * Lotus Land Story has Reimu and Marisa competing to fight their way into the world of dreams, where Kazami Yuuka resides. Reimu and Marisa also visit another dimension that isn't very well explained in the extra stage, just to beat up the creators for no reason at all.
 * Mystic Square features the heroine of the player's choosing invading Makai, the world of demons.
 * Perfect Cherry Blossom features the Netherworld, and Hakugyokuro, land of restless spirits, and effective equivalent of Purgatory. It also has the legendary lost land of Mayohiga, home of the Yakumos.
 * Yukari Yakumo, a Reality Warper, deserves special mention, as her power of "Borders" allows her to generate portals to any other location or dimension she can conceive of. She can even seemingly create new dimensions, by manipulating the boundary between fantasy and reality, and so make a dream or thought into a real dimension, and then open a portal to it. Needless to say, this makes her a Physical God for all intents and purposes.
 * Imperishable Night has... the Moon. Turns out, there's an extremely different world underneath all that barrenness - a highly advanced civilization hiding itself from the "impurities" of the Earth. Revisiting the Moon also becomes the objective in Silent Sinner in Blue.
 * Phantasmagoria of Flower View has the Sanzu no Kawa, a mythical, River Styx-like river that separates the land of the living from Higan, the land where the dead wait for their judgment, and, as a result, can wind up in Heaven, Hell, or get reincarnated.
 * Subterranean Animism has the Hell of Blazing Fires lying in the cave networks beneath Gensokyo and the Earth in general, presumably in some sort of vague pocket dimension in the Earth's mantle that only appears in Gensokyo version Earth.
 * Undefined Fantastic Object has Makai (although some debate as to whether it is the same Makai from before, there's some evidence that it is.) Byakuren was sealed within Makai for trying to unite humans and youkai (and getting hordes of racists on her ass), but is released thanks to the result of her followers' actions.
 * The Nicktoons Unite! series. The world of every Nicktoon show is actually a dimension in the Nicktoon multiverse.
 * Kingdom Hearts indulges in this concept with the Worlds; separated by near-impenetrable barriers(Unless you can find a way to bypass it), but all sharing the same oceans, skies. Unlike most examples, the Worlds seem to be in one universe instead of scattered across infinite dimensions.
 * Eternal Darkness: After beating the game under all 3 ancients you find out that the other ancients (the ones that you beat in the previous two playthroughs)
 * Though not officially stated, the Rift from Final Fantasy V seems to be a wall seperating all the various universes of the Final Fantasy games. Only Gilgamesh has been shown to be able to get around it freely so far. After his banishment there by Exdeath, and his subsequent Heroic Sacrifice against Necrophobe, he sets off on a quest to collect rare weapons, appearing in the worlds of Final Fantasies I (remake), IV, VI (remake), VIII, IX, and XII. At some point, he also manages to find his way into the conflict between the gods Cosmos and Chaos, and is overjoyed to find Bartz there, having been itching to get a rematch against him. Unfortunately, Bartz has only the faintest of memories of his home world in this continuity, making ol' Gilgamesh an Unknown Rival to him.
 * Anachronox takes place in one universe but ; if there was a second game, we would have gone to the former. Plot:
 * The Perpetual Testing Initiative DLC for Portal 2, this is the Excuse Plot. Since Aperture Science is bankrupt, all test chamber construction has been outsourced to Aperture's in other universes, which is then tested in and stolen back. Comes with new Cave Johnson audio that gives us such gems as Space Prison Warden!Cave, Evil!Cave, and Hobo!Cave.

Web Comics

 * All Over the House is linked to The Life of Nob T. Mouse via portals and random dimensional jumps. As a result, there are in-universe examples of crossover media, such as The Blobland Gang stories, which Tesrin of All over the house read as a child. These are based on Hubert Schlongson's visits to Blob City, which is the main setting of The Life Of Nob T Mouse.
 * Bob and George has a lot of different universes, most of them with completely crazy versions of the characters. (One universe is the universe of the original Japanese Rockman games.)
 * Ultima Java has an interesting example; there are many alternate universes, but each universe has a corridor connecting different worlds within that universe. So, in effect, universes within universes.
 * Sluggy Freelance has introduced over a dozen alternate universes by this point, ranging from slightly tweaked versions of the main universe to an endless void outside time to Hell itself.
 * El Goonish Shive has a multiverse of worlds sharing many of the same characters. Among them is the main EGS-verse, the Alpha Dimension (where Tedd is a misunderstood Evil Overlord type with a cybernetic hand), the Beta Dimension (where Elliot was born female and Tedd wears square glasses), the Second Life universe (aliens got involved in the American Revolutionary War, Elliot was born as Ellen and went to school with Kaoli), and the less-canon AF04 (April Fools 2004) universe (Tedd was born female, Sara is a goth, and ).
 * And oddly, Tedd is in every universe, something that nobody else can claim. Might be Author Appeal or something more considering Shive's love of Chekhov's Gun.
 * The KAMics is a multiverse & even has a company that allows travel between alternate universes.
 * The Life of Nob T. Mouse contains a multiverse of sorts through its Quantum History literary device, where each point in time is a parallel universe that may or may not be attached to any other point in time. Although originally devised as an excuse to get around continuity errors, it allows multiple versions of the main characters to exist at once and have different adventures at the same time.
 * The Walkyverse has several, but most specifically: the Dargonverse, where Jason's Father's Organization was before it came to the Walkyverse; the NoWalkyverse, where Walky went to college and never joined SEMME; the Dumbiverse, where all of the Walkyverse characters went to the same college and SEMME and aliens don't exist (named for the comic revolving around it, Dumbing of Age), and the Fans universe, which is...well...yeah. There's also one where Sal and Danny ended up together and had a kid, and a few others that have gone unnamed.
 * French webcomic Dragon Ball Multiverse: An alien race discovers inter-universal travel and gathers the best fighters from every universe to compete in the ultimate tournament. Some of these fighters include a Vegetto that remained fused, a Cell that defeated the Z-Warriors and a Kakarot that didn't lose his memory.
 * Homestuck has a multiversal setting, with the kids' universe and the trolls' universe playing a major role in the plot.
 * Scenes From a Multiverse, a comic about life in an ordinary multiverse.

Web Original
"Puppetverse 20X6 Old-Timey Peasant Universe Teenage Universe Retro Video Game Universe Sweet Cupping Cakes Universe Storybook World"
 * The League of Intergalactic Cosmic Champions was a multiverse. A few of the other universes were other cybersoaps on Nitcentral.
 * In Homestar Runner, there are a lot of alternate universes. Just to name a few...


 * In one Strong Bad Email, The Cheat builds an alternate universe portal that sends Strong Bad to several of the alternate universes (in other words, a flurry of in-jokes).
 * In the fifth episode of Strong Bad's Cool Game For Attractive People, Trogdor manages to escape from the video game universe into Free Country, USA.
 * Suzumiya Haruhi no Yaku-Asobi adds a multiverse (and sliders) to the list of crazy things caused by the incident "three years ago". At least two slider factions consist of empires that span multiple dimensions (one of which officially encompasses around 500 and is ruled by an expy of Saber).
 * Protectors of the Plot Continuum treat every fictional continuity and canon as its own universe within a multiverse. There are several such multiverses, many being alternates of the one the PPC looks over.
 * Dominion and Duchy is explicitly described as taking place in a multiverse. Which is at the centre of multiple multiverses held together by something called the Framework of Reality. The universe the story takes place in holds the object that all the different multiverses orbit around.
 * It also implies that there is trade, as in the epilogue, a huge corporation makes a "Dimensional Elevator".
 * A staple of LiveJournal coms like Sages of Chaos, Dear Multiverse, The Lunatic Cafe, and Dear Mun.
 * The Atop the Fourth Wall / The Spoony Experiment amalgam seems to have this property, most easily accessed by forcing someone to review some part of the Ultimate Warrior series of comics, which are so awful they break reality. In the course of both sets of reviews, Linkara and Dr. Insano flash through a long series of alternate universes, depicted as female/hippy/plushie/reversed versions of themselves/bad actors/etc. Dr. Linksano purposefully uses this to
 * Little Lenny Penguin And The Great Red Flood references a multiverse frequently. Word of God says that this multiverse is "anywhere where space and time exists", which is 99% of pretty much everywhere, broadening the concept to disturbingly confusing new heights.
 * Really, what would The Multiverse be like without Jenny Everywhere?

Western Animation

 * Futurama did this, and we discover a world where coin flips always have the opposite results—as a result, Leela and Fry are married, Bender is gold-plated instead of his usual color ("Bite my glorious golden ass!"), etc. As it turns out, both universes are stored in a box in the other one.
 * They later visit dozens of other universes—ones where they're all robots, or hippies, or Romans...
 * Also, in an earlier episode of Futurama, there was another universe from across the edge of the universe—where the Planet Express crew saw their counterparts dressed as cowboys. At that time, it was said to be the only other universe.
 * Word of God claims in the DVD Commentary that the cowboy one was a parallel universe (thus they're always the same distance from each other but never accessible to each other), the others are perpendicular universes (which means they're accessible to each other but only for a limited amount of time).
 * Family Guy, "Road to the Multiverse". Another Brian and Stewie Road Trip episode, this time going through a Sliders-inspired adventure. Some universes were an Alternate History (where Japanese never surrendered in WWII, the Cuban missile crisis, etc.), while others were the characters done in different art styles (the Disney universe, Robot Chicken universe, political cartoon universe, blocky universe, etc.). There was even a short trip to the real world.
 * The TV movie Turtles Forever establishes that every single version of the Teenage Mutant Ninja Turtles (i.e. every single comic, TV, and film series) all exist simultaneously as separate universes in a multiverse. The Shredder gets rather pissed when he discovers that there are teams of Turtles in each one.
 * The Battle Nexus fighting tournament draws in combatants from all over a multiverse that appears to be a sort of different multiverse concept. Usagi Yojimbo's world is one of them.
 * Justice League: Crisis on Two Earths takes a serious look at this trope to the point of near-deconstruction:
 * Video Land in Captain N is basically a universe for each video game plus the Real World.
 * The end of Spider-Man: TAS featured a team-up of several Spider-Men to prevent Spider-Carnage from destroying a multiverse. One of them didn't have any powers and was really an actor playing Spider-Man in a movie. His universe was strongly implied to be "ours".

Real Life

 * Although the concept of the Multiverse has had a presence in scientific thought for years, it has disturbed real cosmologists because in its basic formulation it is unfalsifiable and thus untestable scientifically - but it was still being treated scientifically by many scientists. Any attempt to work up a testable version of the idea required invoking privileged viewpoints in space-time (which by relativity cannot exist), among other problems. However, a pair of cosmologists working independently each came upon a method of measuring and deducing things about the multiverse that doesn't rely on a privileged point of view; when they combined their work, they discovered that these turned out to be two different approaches to looking at the same things coming at them from two different directions. The upshot of this is that, as of early 2010, multiverse cosmology has just gone from a cool unprovable idea to something close to science fact.