Disney did it again with Wreck-It Ralph. Although all the characters from existing video games usually appear for a short period of time, it still counts.
The Cinema Snob reviewed two crossover films that matched an Old West gunslinger with a classic horror character: Billy The Kid Vs. Dracula and Jesse James Meets Frankenstein's Daughter.
Batman v Superman: Dawn of Justice, which in addition to the obvious crossover in the title, also features a major appearance from Wonder Woman, and cameos from Cyborg, Aquaman and the Flash.
Black Adam (2022) has an appearance from Superman during the mid-credits stinger.
Although unnamed in Doctor in Trouble, the doctor who Dr. Burke chats to early on at the hospital is supposed to be Dick Stuart-Clark from the TV series Doctor in the House. Furthering the connection to the series, Yutte Stensgaard (Eve) played Helga in a few episodes, while Graham Chapman (Roddy) was one of the writers.
Too many Godzilla movie crossovers to list as many of his adversaries (including King Kong and Mothra) had starred in their own films previously. He even had a smackdown with the American Godzilla (from the 1998 film) in Godzilla: Final Wars.
There is an old film, Hercules, Samson and Ulysses which features the two strong men of the bronze age coming to blows. It's also So Bad, It's Good.
In Holiday on the Buses, Arthur Mullard and Queenie Watts reprise their roles of Wally and Lily Briggs from Romany Jones, which was another Britcom by Ronald Chesney and Ronald Wolfe, the creators of On the Buses.
Danny Trejo has stated that the Machete Cortez in Machete is what the Machete Cortez from Spy Kids does when he's not taking care of the kids. As Robert Rodriguez intended to make a Machete movie years before he was able to, he inserted a more family-friendly version of the same character into Spy Kids in the meantime, so this is technically canonical.
The Marvel Cinematic Universe films, starting in 2008, built up a series of increasingly obvious crossovers (using the same actors in almost all cases):
Agent Coulson is a minor character in Iron Man, and Nick Fury has a cameo, where he name-drops "The Avengers Initiative", in The Stinger.
Both play more significant roles in Iron Man 2, where Fury reveals Howard Stark's S.H.I.E.L.D. connection and Coulson gives Tony Stark Captain America's shield.
Black Widow also plays a key role in Iron Man 2.
It is Coulson that finds Mjolnir at the end of Iron Man 2, and contacts Fury about it.
Coulson plays a major role in Thor, and Nick Fury has a brief cameo.
Nick Fury plays a minor role in Captain America: The First Avenger, where he greets a newly-awakened Steve Rogers and invites him to join the Avengers.
By the time they got to Thor and Captain America, the crossovers were pretty blatant, as the Avengers movie was starting production and public knowledge:
S.H.I.E.L.D. plays a major role in both films, and is shown to be obviously "collecting" superhero-related artifacts and information.
Tony Stark's father is a major character in Captain America and gets a name-drop in Thor.
And all of the crossovers up to that point lead to the feature-length Avengers movie (though they presumably will not stop there).
Offsetting the above, Thor's next movie after Civil War, Thor: Ragnarok, has the Hulk/Bruce Banner as a main character next to Thor himself, and also features a brief appearance from Doctor Strange near the beginning.
The View Askewniverse movies are all connected, mostly by Jay and Silent Bob appearing in every movie. Aside from that, there are numerous events, elements, and secondary characters that intertwine throughout all six films. It also helps that the movies take place in the same state. Jay and Bob also pop up in Scream 3.
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