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. 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.
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: Crime Scene Investigation 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, Thats 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.
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 MacGyver 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.
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 now characters from iCarly and Victorious guest starred on Sam & Cat.
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.
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.
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.
In the realm of examples that don't inflict pain in the viewing audience, from Lost Galaxy on, there's a recurring tradition of the seasonal team-up, a two-parter where the previous season's cast meets up with the current Rangers to deal with a threat that requires their combined powers.
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.
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 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 multiple Star Trek series was that involving the Maquis. The reason behind their existence (the creation of the DMZ) was established in Star Trek: The Next Generation Season Seven but the Maquis were introduced in Star Trek: Deep Space Nine Season Two before turning up in "Preemptive Strike", the penultimate episode of The Next Generation, and forming a major part of the backstory of Chakotay and B'Elanna in Star Trek: Voyager. The destruction of the Maquis by the Dominion in Deep Space Nine Season Five comes back to haunt Chakotay and B'Elanna in Voyager Season Four when the ship finally makes contact with the Alpha Quadrant.
Events from one series do occasionally impact on later ones in less direct ways though. For instance, a major part of Sisko's backstory in Deep Space Nine was the death of his wife Jennifer during the Battle of Wolf 359, which occurred in TNG's "The Best of Both Worlds". The subsequent Borg attack on Section 001, which occurred in Star Trek: First Contact, is mentioned occasionally in Deep Space Nine while the Dominion War from Deep Space Nine is mentioned in Voyager's "Message in a Bottle", Star Trek: Insurrection and Star Trek: Nemesis.
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.
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. Though there are occasional glitches when actors who played one character on on show turn up as a completely different character on the other.
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. Here's a handy chart for those keeping track◊. And the chart isn't even the full extent of it, since the list doesn't include ANIMATED shows that have crossed over with these series (notably a brief crossover between The Simpsons and The X-Files) increasing the cross overs to just about 99% of all series ever put on television.