The classic crossover started itself off as a good way to make the best of what you've got — so if you have two shows on your roster, it's a no-brainer to have the shows and characters cross over every once in a while, especially if one of the shows is less popular than its sibling.
The popular way of doing this is the "true" crossover, in which a storyline will begin in one series and cross over into the next one, encouraging viewers to tune into a show that may be thematically similar but which they do not usually watch.
For example, Buffy the Vampire Slayer has a handful of episodes where the characters head off at the end, only to turn up in Angel straight after. This also works the other way, with a magic amulet in Angel turning out to be vitally important for the last-ever episode of Buffy. This also highlights one of the dangers of crossovers; if you or a fan watched only Buffy the Vampire Slayer, but not Angel, this particular amulet appeared to come totally out of nowhere to save the day... (See Red Skies Crossover.)
Alternatively, a single popular character can cross over from one show into an other for a brief guest appearance; this has the effect of attracting that character's fans from the other show without requiring the writing teams to sync up or creating DVD- and arc-unfriendly episodes. This is very common in comic books, in which most characters are part of a larger universe, such as the Marvel Universe or The DCU. It happens less often in TV and movie properties based on comic books, since they are often made by different production companies.
As more ways of developing connections between shows formed, we ended up with lots of different subtropes of crossover. All of these can be seen on the Crossover Index and any examples for those will be found on their own pages.
- Anime & Manga
- Comic Books
- Fan Works
- Live-Action TV
- Professional Wrestling
- Tabletop Games
- Video Games
- Web Original
- Western Animation
- The 2011 spring animation festival for CCTV Dream has popular characters from Chinese animation watch and interact various animated shorts from different studios.
- Due to the popularity of Arthurian legends, many unrelated Welsh and Breton folk tales later had some Arthurian elements and trappings added, such as with Tristan and Iseult.
- Older Than Feudalism: The Classical tale of Jason and the Argonauts brought together many mythological heroes in a sort of Justice League Ancient Greece. "Hero" in Classical Mythology was an awfully vague term (as Larry Gonick put it, heroes aren't necessarily heroic, just excessive), and since the mission was to steal an item (the Golden Fleece) "Justice League" isn't a terribly good name for them....
- Maid Marian, the shepherdess, featured in many pastoral plays popular at May festivities. Then there was a fad for Robin Hood plays. Then someone put the two characters together. It worked so well that Maid Marian is best known as a Robin Hood character, even though her independent existence predates him.
- Batman and Robin made frequent appearances on The Adventures of Superman.
- The Archers had a crossover with Gardener's Question Time for Easter 2011, where the real-life panel, chaired by the real-life Eric Robson answered questions from the population of Ambridge.
- On the April 1, 2013, broadcast of "Family Life Today", John Fuller and Jim Daly, from "Focus on the Family", crossed over.
- Two episodes of The Goon Show featured guest appearances by Jack Train as Colonel Chinstrap from ITMA, a popular comedy show from the previous decade. There was also a crossover with the comedy "Educating Archie", called "Archie in Goonland".
- An arguable one, similar to the Comic Relief Panel Game crossovers above, the New Year 2010 episode of The BBC Radio 4 comedy panel game The Unbelievable Truth featured Stephen Fry, Alan Davies, regular QI panelist Rob Brydon and QI producer John Lloyd. The format of the game was unchanged, but the "obvious answer" klaxon was added. Much to the satisfaction of the other players (and host David Mitchell), Stephen set it off twice.
- Godzilla vs. Evangelion: The Real 4-D at Universal Studios Japan is Exactly What It Says on the Tin: Godzilla against the cast of Neon Genesis Evangelion—more specifically, the Godzilla of Shin Godzilla vs. (given Asuka being identified as "Shikinami", not "Soryu", and the designs of the EVAs) the cast of Rebuild of Evangelion.
- LEGO has many licensed sets of Star Wars, Harry Potter, superheroes, etc. Easy crossover potentials, as demonstrated with The LEGO Movie. Especially with LEGO's own original sets like Ninjago and Legends of Chima.
- Mr. Potato Head has outfits from many other franchises. Transformers is a given, since Hasbro owns both, but the Heads can also dress up as characters from Star Wars, DC, and Marvel.
- In Maji de Watashi ni Koi Shinasai!, members of the Kuonji household shows up in several places, as they live in the neighbouring area. In one of the routes they even spend the night in that mansion. Naturally, one of them mentions that Yukie has a nice voice.
- Despite being an internally consistent multiverse, the Nasuverse only has crossovers in incredibly round-about ways. Part of the problem is, well, Word of God stated that some of the characters simply cannot exist in the same universe for Canon reasons.
- Zelretch was seen in the Prologue to Tsukihime with a young Arcueid. He also appears for one scene in the "Heaven's Feel" route to Fate/stay night, walking in from Another Dimension to meet Rin.
- Aoko Aozaki spends time in the beginning of Tsukihime with a young Shiki Tohno, appears later in Melty Blood, and is the main character of Mahoutsukai No Yoru. Her sister, Tohko Aozaki, is the Older and Wiser mentor figure of the (arguably) Tsukihime-prototype, Kara no Kyoukai, plays a major role in Mahoyo, and is mentioned in an ending of Fate/stay night. Despite each of them knowing a different Shiki (each possessing the Mystic Eyes of Death Perception) it's explicitly stated that Tohko and Arcueid cannot exist in the same universe (and most likely, Shiki Ryougi and Shiki Tohno can't, either).
- Shirou's employer at his part-time job in Fate/stay night (who appears once in an easily-missed scene) shares a name with a drug dealer in Kara no Kyoukai.
- Cornelius Alba of Kara no Kyoukai makes a cameo appearance in the Fate/Zero anime adaptation, though he doesn't appear in the original light novels.
- Sion's magic gun in Melty Blood is a replica of Godo's Black Barrel in Angel Notes, which Ciel is reportedly carrying around in Tsukihime.