The first crossover, with Thor: The Dark World, was heavily promoted, only to turn out to be a Red Skies Crossover. While it dealt with Asgardians, it was completely unconnected to the movie except for a single scene at the beginning.
The second crossover involved a cameo from Lady Sif. It wasn't advertised as a direct crossover like the previous episode, but the plot was still slightly more connected to the movie.
The crossover with Captain America: The Winter Soldier was also heavily advertised, but by this point the audience wasn't expecting anything special. Turns out they were wrong—the movie had massive effects on the show, involving a complete Re Tool following S.H.I.E.L.D.'s fall in the movie, and spending the next season and a half dealing with the fallout from the events. The show made excellent use of its lower-tier nature to address the issues that the movie didn't have time for.
All My Children, One Life to Live, and the defunct Loving were all created by Agnes Nixon and share a universe. Numerous crossovers have occurred over the years. In 2004, AMC and OLTL launched an ambitious crossover storyline in which a baby from All My Children was kidnapped to One Life to Live. The story lasted several months. Traditionally, ABC considered all of their soaps to share a universe, which allowed for the 1999 story (essentially a ratings stunt) in which Linda Dano's One Life to Live character traveled to All My Children, Port Charles, and General Hospital searching for her long-lost daughter, who turned out to be a fellow soap-hopping character, Skye Chandler.
Especially between Port Charles and General Hospital, which were set in the same town and hospital (the former show was a Spin-Off of the latter) . Characters from one show frequently appeared on the other, and major eye-candy events like the annual Nurses Ball would start on Port Charles and finish on General Hospital. Only when Port Charles took on a more supernatural tone did the crossovers stop.
Similarly, on CBS, The Young and the Restless and The Bold and the Beautiful were created by William Bell, leading to many crossovers and shared storylines, the best example being the former show's resident psycho, Sheila Carter, being presumed dead and thus a Karma Houdini for her various crimes turning up on the latter show to wreak much of the same havoc. However, there have been occasional glitches when actors who played one character on one show turn up as a completely different character on the other.
Other soap examples have even crossed networks. As the World Turns and Another World were on CBS and NBC, respectively, but when the latter show was canceled, a handful of its favorite characters were brought to ATWT for several years.
The continuity was thrown for a loop when Peter MacNicol played a one-off character on Boston Legal, a spin-off of The Practice, after having played a lead character on Ally McBeal.
And for another loop when the appearance of Bill Smitrovich as a guest star on Boston Legal, in a different role from the cranky, vindictive D.A. he played on The Practice.
The role was Not So Different at all. The main difference was that his Boston Legal character worked in a state where Death Penalty was applicable.
The most baffling loop-thrower has to be John Laroquette, who had a notable guest appearance as a devious client on The Practice before showing up as regular character Carl Sack in Boston Legal's fourth season.
A 1968 Thanksgiving episode of The Beverly Hillbillies had that show's characters traveling to Hooterville, leading to a three-way crossover between The Beverly Hillbillies, Petticoat Junction and Green Acres. Earlier episodes of Green Acres had shown Hootervillians watching the fictional Clampetts on TV and even producing an amateur play based on The Beverly Hillbillies TV show.
Breakout Kings: In the third episode of the first season, Prison Break's T-Bag....breaks out from a prison. An unprecedented case of a character crossing over to a show on a completely different network but not so surprising considering both show have the same producers (and an obvious common theme).
Naturally, the shows did it several times, especially in Season 4 of Buffy / Season 1 of Angel . However, this did not happen during season 6/3, when Buffy's Channel Hop made it a lot harder and they were essentially banned. Then the ban was lifted, and crossovers happened again.
One interesting example was the Buffy episode "Fool for Love", and the immediately following Angel episode "Darla", which didn't share an A Plot, but were linked together by a series of flashbacks filling in Darla's, Angel's, Drusilla's, and Spike's backstories (including one flashback shown first from Spike's perspective in "Fool for Love", and then Once More with Clarity from Darla's perspective in "Darla").
In the Season 8 comics there is a crossover with Fray in Time of Your Life. Future Willow transports Buffy to the future.
Likewise, Fred Rogers appeared on Sesame Street, and Big Bird appeared on Rogers' show in the Neighborhood of Make-Believe.
Casualty and its spin-off Holby City have had four crossovers titled Casualty@Holby City (although they were given a special title, they ran as two-parters in the usual slots): two Christmas episodes, one Halloween Episode, and a Very Special Organ Donation Awareness Week episode. We have yet to see Casualty@Holby City With Holby Blue, but doubtless it's coming...
And it has: although it wasn't called Holby Blue@Holby City, the first episode of Season Two of Holby Blue was a direct continuation of that week's Holby City, both of which featured the medics and the cops.
CSI has crossed over with both other shows within the CSI franchise, CSI NY,CSI: Miami, as well as Without a Trace. Miami, meanwhile, has crossed over twice with CSI NY, and New York once also did so with Cold Case, and the three core CSI programs have crossed over with a three-part story featuring Ray Langston. All five are produced by Jerry Bruckheimer and air on CBS, so bringing them together is less complicated than most.
Colbert and Jimmy Fallon each had the other crossover in the promotion of a fundraiser competition. It culminated in a performance of Rebecca Black's "Friday" that contained nearly the legal limit of awesome.
All of the series in the DCLAU tend to have numerous crossovers with eachother. In 2006, That's So Raven, Hannah Montana, and The Suite Life of Zack and Cody have crossed over (in a special called That's So Suite Life of Hannah Montana), and, in 2009, there was an another with Hannah Montana, The Suite Life On Deck, and Wizards of Waverly Place (Wizards on Deck With Hannah Montana). Interestingly enough for shows with little enough continuity, the characters from The Suite Life and Hannah Montana recalled the previous crossover.
The character of Captain Jack Harkness first appeared in Doctor Who near the end of the 2005 series. At its finale, he was left behind by the Doctor, and was next seen in 2006 starring in his own spinoff, Torchwood. Torchwood ended its first series with Jack running away to rejoin the Doctor, which occurred near the end of Doctor Who's 2007 series in the three-part finale, after which Jack headed back to his team for Torchwood's second series.
Jack returns for the series 4 finale, along with Ianto and Gwen. The episode crosses over even further with the return of Sarah Jane — accompanied by her adopted son from her own spinoff series.
Likewise, the Doctor has had a guest appearance in The Sarah Jane Adventures, as has the Brigadier (making him the first Classic Who character to appear in a Nu-Who spinoff, but not Nu-Who itself). Another SJA crossover aired, with Eleven and Jo Jones.
The short video "Wholock" has the Eleventh Doctor show up outside 221B Baker Street to invite Sherlock to go with him. Prior to that, we are shown a wall full of photographs of the Doctor and the TARDIS and newspaper clippings. Basically, the Doctor is the only guy who utterly baffles Sherlock. As the TARDIS is dematerializing, John runs outside screaming Sherlock's name.
ER crossed over with Third Watch. Unsurprising, as they were both created by John Wells and have similar subject matter. Unfortunately, it ended up negating a throwaway line in which a Third Watch character mentioned watchingER on television (obviously a Shout-Out, but it would mean that the two shows didn't have a Shared Universe).
Eureka and Warehouse 13 have a pair of crossover episodes, where Fargo upgrades the Warehouse's computer system and Claudia visits Eureka. Fargo even managed to MacGyvering a lightsaber with a laser pointer and an artifact. SyFy even used these episodes for a canon crossover ship. Claudia/Fargo. Alphas was also given a crossover with Warehouse 13, although no character in Alphas has yet appeared in either of the others.
In the early 90s, an episode of Family Feud had some cast members of The Price Is Right compete on a Celebrity Edition (namely then-host Bob Barker, then-announcer Rod Roddy, and three of the models). The first episode of the week was treated as a crossover, with Feud announcer Gene Wood telling Bob and co. to "come on down" as if they were Price contestants (the shows taped in the same studio at the time).
The Fox show The Finder, besides having been given a Back Door Pilot from Bones, later had a crossover from the same when Sweets popped up to do a psychological evaluation of Walter. Dr. Jack Hodgins appeared in an episode of The Finder.
Multiple characters from Cheers dropped by on Frasier. The producers said the they moved Frasier to Seattle to avoid this, but couldn't hold out forever. The only character who did not appear on Frasier was Rebecca Howe and this was due to actress Kirstie Alley's refusal to participate on the grounds that her religion (Scientology) looks down on psychiatry.
Lilith was the first and the only one to recur. Justified in that Lilith was the mother of Frasier's son, and therefore more a part of the Frasier-verse than the Cheers gang.
Frasier's son Frederick appeared about once a year, played by two different actors, neither of them the "Frederick" from Cheers.
Sam Malone (Season 2)
Diane Chambers (Twice in dream sequences, once in a season 3 makeup episode with Reality Subtext - Shelley Long and Kelsey Grammer had an on-set rivalry on Cheers, but they made up in the course of filming the episode.)
Woody Boyd (Season 6)
Cliff, Norm, Carla, and a bunch of the Cheers regulars (Season 9, "Cheerful Goodbyes").
Friends with Mad About You, somewhat, as in a few eps Ursula can be found as the Buchmans' regular waitress, much to their dismay.
There was an actual crossover when the MAY characters appeared in the coffee shop, mistook Phoebe for Ursula, and, assuming Ursula had been (rightly) fired there and hired here, tried to order coffee from her.
There was also at least one with The Single Guy.
A more straight example would be the crossover with Caroline in the City in the episode "The One With The Baby On The Bus". Caroline meets Chandler and Joey on the street and mistakes them for a gay couple. The same night that episode aired, Chandler appeared in an episode of Caroline.
Then there was NBC's "Blackout Thursday," in which the characters on Mad About You caused a citywide blackout, followed by the characters on Friends and Madman of the People dealing with the aftermath. Notably, Seinfeld didn't join in as its script for the week had been written long before.
The Golden Girls, Empty Nest, and Nurses were all made by the same production company and all shared the same Miami setting, so there was predictably a fair amount of crossover among them.
Shonda Rhimes' Grey's Anatomy and Private Practice have crossovers seemingly every five episodes or so. Now that Rhimes has developed a new show, Scandal, one wonders just what kind of menage a trois is in store.
Happy Days and Laverne and Shirley had characters appearing on each other's shows fairly regularly, and there was one true crossover storyline that spanned episodes of both series — which was only reasonable, since Laverne And Shirley was a Spin-Off of Happy Days. And just to add a little weirdness to the mix, Mork and Mindy was also a Happy Days spinoff...
In "iStart a Fanwar," Eric, Craig, and Gavin from Drake & Josh make an appearance, along with Stacey Dilsen from Zoey101.
And Sam & Cat had appearances by characters from iCarly and Victorious (Freddie and Gibby from the former, and Robbie, Jade and Sikowitz from the latter).
Kaizoku Sentai Gokaiger, the 35th Anniversary Super Sentai series, features characters who can use the powers of the 34 previous teams, and at least one member of most of the previous teams have appeared on the show.
Kamen Rider Decade, itself an anniversary crossover of the previous 9 Kamen Rider titles, became the first Kamen Rider series to cross over with a Super Sentai series, with Tsukasa and Daiki appearing in two episodes of Samurai Sentai Shinkenger, and the Shinkenger appearing in an episode of Decade. This, in turn, was followed by...
The episodes of Law & Order: Special Victims Unit and Law & Order: Trial by Jury titled "Night" and "Day," respectively, were crossovers of each other on the same night. The major characters from each series appeared on both episodes, which dealt with a rapist. SVU focused on the investigation and arrest of the rapist, while Trial by Jury focused on the trial. (There was some discontinuity here. At the end of SVU, the case was declared a slamdunk. At the beginning of TBJ, the case became a sure loser.)
Law And Order: SVU did this years ago with a 2-part event, "Entitled," the second half of which took place on the original Law & Order. However, this plays havoc with syndication, because the shows are rerun on different networks in America; the SVU episode leaves the viewer hanging without the second half and the mothership episode confuses those who didn't see the first and wondering why it seems to be starting In Medias Res.
Law & Order also crossed over with Homicide in what was originally a two-parter. While working together on a single case, the two squads had personality clashes, and normally-infallible characters faltered in unfamiliar settings. In a good move, when this episode is rerun TNT will show both the L&O and Homicide episodes back-to-back, even though they don't usually show Homicide at all.
In The Man from U.N.C.L.E.'s heyday an episode of Please Don't Eat The Daisies had the children believing their father was a spy after seeing him pass a matchbook to Illya Kuryakin in an innocuous (or was it?) encounter. Hilarity ensues until, in the show's concluding scene, someone is brought in to convince the children that they've jumped to a foolish conclusion. It is Robert Vaughn whom the kids recognize instantly as Napoleon Solo.
Martial Law has the appearance of Chuck Norris' character for a couple of episodes. In fact, for that case, you have to watch the continuation of the case in Walker, Texas Ranger if you want to know how it is resolved.
USA Network had a set of commercials crossing over their various shows; Adrian Monk and Johnny Smith are each thankful the other doesn't want to shake hands; Johnny and Shawn Spencer debate who has the worse past; until they notice Monk counting his food; etc.
Back when USA Network showed Lorenzo Lamas in Renegade and Highlander The Series, a carefully filmed Duncan Macleod and Reno Raines exchanged quips about their situations; Duncan is 400 years old, Reno has some socks that are over 20 - till it ends with Duncan's katana slashing the front wheel on Raines' motorcycle.
CBC time-twisting crossover: The Murdoch Mysteries episode "Republic of Murdoch" features a 19th century "Jacob Doyle". And the the Republic Of Doyle episode "If the Shoe Fits" features a 21st century "Bill Murdoch".
Kensi from NCIS: Los Angeles made a guest appearance on the revival of Hawaii Five-0. And then NCIS: Los Angeles and Hawaii Five-0 did a two-way crossover where Callen and Sam came to Hawaii to prevent a black-market bioweapons buy, then Danno and Chin Ho went to L.A. to stop one of the involved parties from unleashing the bioweapon in question.
Pride, LaSalle, Brody, and Percy appear in the first part of the NCIS/ NCISNewOrleans two-part crossover event "Sister City" and Gibbs, Ducky, Palmer, Abby, and Bishop appeared in the second part.
And Bud Roberts from NCIS' parent series JAG appeared in one episode in the first season.
Retired Rear Admiral A.J. Chegwidden is Gibbs's lawyer in the NCIS Season 10 finale.
The Neighbors recently crossed over with Shark Tank, to kick off a story arc featuring one of the Sharks investing in Debbie's quest to go to business school.
A rather famous "Night of Elizabeth Taylor" also tied The Nanny to the other shows on the network that night, Can't Hurry Love,Murphy Brown, and High Society.
Power Rangers usually has the trend to have the previous team appear for a crossover special. This first appeared on Power Rangers Zeo, in which the Alien Rangers teamed up with the Zeo Rangers, but more or less became regular 3 seasons later, during Power Rangers Lost Galaxy. Other notable crossovers include:
There was also a three-part episode in the third season of MMPR that featured the main character of of the Masked Rider series. Interestingly enough, the original Japanese franchises Super Sentai and Kamen Rider, which Power Rangers and Masked Rider are based on, never crossed over until 14 years later in 2009, when Kamen Rider Decade crosses over with Samurai Sentai Shinkenger.
The fourth season of Once Upon a Time is one lengthy crossover with Frozen. Though the very nature of the show itself is one between almost every popular fairy tale that Disney had explored.
Word of God says that if Psych hadn't made Leverage a fictional show within Psych's universe, ''Leverage's Eliot would have had an uncle named Henry.
"X-Cops" is the 12th episode of the 7th season of The X-Files which made a brilliant crossover with COPS. Shot in the standard COPSReal Time-documentary format, the episode follows Mulder and Scully collaborating with LAPD as they track down an entity that takes the form of your greatest fear.
A Saturday Night Live sketch from 1999 involved a humorous crossover between Seinfeld and Oz, in which Jerry Seinfeld is incarcerated at Oswald State Correctional Facility, where he casually chats about prison life with Tobias Beecher, Vernon Schillinger, Ryan O'Reilly, and Augustus Hill.
There hasn't been a direct crossover between The Shield and Sons of Anarchy, but a gang called the One-Niners, based in Los Angeles, has appeared in both shows, indicating that the shows share a continuity. Considering that one of Sons of Anarchy's producers was a writer on The Shield, it's not surprising.
And you can tack on The Unit to that as well, which also shared a producer with The Shield and had a gang-in-common, in this case the Byz-Lats.
The Six Million Dollar Man and The Bionic Woman had regular crossovers, with two-part and three-part stories involving both shows (most notably "Kill Oscar," which had parts 1 and 3 on Jaime Sommers's show with the middle part on Steve Austin's). Curiously, although both series aired on ITV in Britain, the crossover episodes weren't treated as such: all three parts of "Kill Oscar" and both parts of "Welcome Home, Jaime" were aired as episodes of The Bionic Woman, while both parts of "The Return Of Bigfoot" were shown via The Six Million Dollar Man.
Stargate SG-1 and Stargate Atlantis have had several instances of crossovers. Among the most notable are the characters Rodney McKay and Elizabeth Weir, who were introduced as secondary characters in SG-1 and became two of the first regular characters on Atlantis (later joined by other SG-1transplants.) Often times, things that happen in one series have an impact on the other, such as the recovery of a ZPM by SG-1 helping to defend Atlantis during its siege.
There was a dedicated episode where SG-1 goes to Atlantis to find out the secrets of the Deus ex Machina that'll save the galaxy, in the process both teams save the other one from disaster by the series' two Big Bads. Additionally, several episodes of Atlantis have included scenes set in the SGC.
The various Star Trek series saw many of these, beginning with Star Trek: The Next Generation, although events in one series rarely affected the others. The crossovers became more frequent in later years.
The only storyline to play a major role in all three "Next Gen" series was the rise and fall of the Maquis. The reason behind their existence (the creation of the DMZ) was established in Season Seven; the Maquis were introduced in DS9 Season Two ("The Maquis") before turning up again in "Defiant" (Jonathan Frakes guest-stars as Will Riker's rebellious clone from TNG, Tom Riker) and several episodes following that, and formed a major part of the backstory of Chakotay and B'Elanna in Star Trek: Voyager. The destruction of the Maquis by the Dominion in DS9 Season Five ("Blaze of Glory") comes back to haunt Chakotay and B'Elanna in Voyager Season Four when the ship finally makes contact with the Alpha Quadrant.
No less than George Takei took part in VOY's Flashback, which builds on Tuvok's service onboard the U.S.S. Excelsior under Sulu's command. Tuvok also appeared in a mirror universe episode and interacted with the DS9 principles for a scene.
Joe Ford: Whilst is nice to see a crossover of DS9 and Voyager, the only actor who isn’t let off the reins in Through the Looking Glass is Tim Russ, which figures given the show he has been leased from. This is the dull old Tuvok that sours the sister show.
The events of First Contact formed the basis for a Star Trek: Enterprise episode, "Regeneration", which ties neatly (sort of) into the Borg's first appearance in TNG's "Q Who".
Tinsel was crossed over by Jacob's Cross during the brief engagement between Phillip Ade-Williams and Chi-Chi Nwachukwu-Abayomi.
Top Gear crossed over with Ground Force for Sport Relief calling the one-off Top Ground Gear Force. They ended up destroying Sir Steve Redgrave's garden in typical Top Gear fashion.
Ultraman and Kamen Rider had a crossover short in the early '90s, unexpected considering each series is from a different company.
Ultraman Mebius has crossovers with older/the original Ultra Series throughout its run. Be it from classic monsters to the older heroes showing up for a team up. The Movie, Ultraman Mebius and the Ultra Brothers takes this to 11.
The WB once had a night dedicated to crossing over their shows. A character from one show would pop up in another, mostly for laughs and night contribute much to episode's plot. It was parodied in that night's episode of Unhappily Ever After, wherein Jackee Harry and hinted that The Wayans Bros. were "waiting in the car".
The Polish Crime and Punishment SeriesW-11 and Detektywi made at least one such crossover: an episode on Detektywi was the first part, depicting the eponymous detectives investigating a case; then, for the second part, the policemen of W-11 took over.
8 Out of 10 Cats does Countdown (twice) and Deal or No Deal (once — and it's the first DoND UK episode not hosted by Noel Edmonds).
The "Tommy Westphall Universe" (or, alternatively, "The John Munch Principle") ties dozens of television series from the 1960's to the present through the characters of Tommy Westphall from St. Elsewhere (who is revealed to have imagined the entire series in the final scene of the final episode) and John Munch, the Baltimore detective who has made crossover appearances in many television series (including the Law & Order canon, The X-Files and Homicide: Life on the Street). Thanks to Munch's crossovers and cameos, as well as shared elements and names that occur between different franchises and series, a theorem was developed showcasing the connections. More than 90% of all television shows are connected via crossover, including the entire All in the Family franchise, The Wire, the entire Law & Order franchise, every major medical drama made in the 90's and 00's, and many more. As of Disney Channel's 2015 Monstober event, the DCLAU is the newest inclusion to this hypothesis, via Girl Meets World (a spinoff of Boy Meets World, one of the shows said to be linked via the hypothesis) being one of the shows to cross over with other Disney shows.
Cannon (from the detective show of the same name) guest starred in the pilot for Barnaby Jones - the two series later had a two-part crossover in "The Deadly Conspiracy," which began on Barnaby Jones and finished on Cannon.
On a flashback episode of Green Acres set during World War II, Eddie Albert's character, about to be sent as an operative behind enemy lines, is told, if he fears capture, to seek out Stalag 13 and be rescued by Hogan's Heroes.
And now Bones appears to take place in the same 'verse as Sleepy Hollow, meaning viewers have to be doubly worried about long-dead bodies having a chance of suddenly getting up and walking. Oh, and that one time Booth thought he saw a ghost can now be seen in a new light. To be fair, though, the Bones side of the crossover had nothing supernatural, you know, besides the presence of a man who was born centuries ago.
The first season of Supergirl featured a crossover with The Flash, (and by extension, Arrow, Vixen, Constantine, and Legends of Tomorrow). This was foreshadowed by a vision of Supergirl in an episode of The Flash, when Barry's is crossing from Earth 1 to Earth 2. In the crossover episode, Barry accidentally ends up in Supergirl's world, after attempting dimensional travel again. One of the themes of the episode is his attempt to find a way home to his own world, while helping out Supergirl battle her enemies. After first encountering her, Barry drops the names of several heroes and villains (including Green Arrow, Firestorm, and Zoom) and is dismayed to find out that no one on this world has ever heard of either them or him. He himself is amazed to discover that aliens are real. He ends up helping to retrofit the city's prison, allowing it to contain individuals with superpowers. He then has Supergirl help him reach the necessary speed to perform a dimensional jump and then throw him through the breach (which he knew thanks to previously helping Eobard Thawne return to his own time).