Additionally, last year Teen Titans Go! had an episode where Robin dressed up briefly as Freakazoid, but this weekend Freakazoid is making an actual appearance on the show, with many members of the original voice cast returning, which presumably means their characters are also going to be appearing
.

During the 20th century the two biggest UK comic publishers were DC Thomson and IPC (also known as/connected to Fleetway, Amalgamated Press, etc.). We've seen links within their respective companies - characters from one DC Thomson humour strip routinely show up in others, and similarly IPC strips weren't adverse to crossovers in company, a trend that has restarted now that IPC properties all belong to Rebellion. And we've seen IPC crossover with US titles on a tiny number of occasions, most obviously via Judge Dredd, thus linking them into the wider Omniverse. But until today the only link I knew of between IPC and DC Thomson was via the Comic Relief Comic, a charity release that was thus able to get permission to use characters from multiple companies, thus allowing Marvel's Captain Britain, IPC's Judge Dredd and DC Thomson's Desperate Dan to all share a panel:

However, now I know of another crossover between the two UK rivals. 

Scream Inn was a series that debuted in IPC's Whoopee!#1 The titular inn was a haunted hotel that offered a cash prize to anyone who managed to stay an entire night while the resident ghosts and ghouls tried to scare them out. During its lengthy run many other IPC characters guest starred, including Frankie Stein and Sweeney Toddler, both themselves guilty of numerous other crossovers. The Innkeeper of the Scream Inn also showed up as one of the Shiver Givers in Shiver and Shake, tying him to yet more characters including Grimly Feendish, another crossover nexus. The point being that any link to Scream Inn from outside of IPC is a doorway to IPC in general.

In DC Thomson's Dandy#2502, cover dated 4th November 1989, we got just that when Smasher visited a haunted house in his own strip:
Money had clearly become tighter for the Inn by 1989, twelve years after they had vanished from the pages of IPC titles, as they'd reduced their prize money, but nevertheless, compare the poster style:
But in case there's any doubt, here's what the inn looked like from a decent outside vantage:
and here are the staff:
and here's Smasher leaving in the morning after winning his prize:

Here's a very early case: In Dumbo (1941), the circus train while going up a steep mountain says "I think I can, I think I can", thus implying that the train is The Little Engine that Could, which at the time had only been seen in books.

RWBY/JUSTICE LEAGUE (DC, 2021) 7 issue miniseries

Loki wrote:

Apropos of nothing, and not many will get it, but still...

Only reason I know is that Sci-Fi Channel used to re-run so many old sci-fi shows back in the 90's.  One's The Invisible Man, and the other's The Gemini Man, two old shows about men who could turn (or were) invisible.  Although I can't remember which one was which.

An old link but one I only just learned about.

Charlie Chan is linked to Mr. Moto through Chan's "Number One Son" Lee Chan (actor Keye Luke). Keye Luke played Lee Chan in eight movies in the 1930s alongside Swedish actor Warner Oland as Charlie Chan. They were partway through filming their ninth movie together, intended to be called Charlie Chan at Ringside, when Oland took ill and ultimately died. Wanting to salvage as much of the shot footage as possible the studio modified the script to make it an entry in another of their series, the Mr. Moto movies. That resulted in Lee Chan remaining a character in the movie, and they even included dialogue where Moto acknowledges Chan, stating he is only an amateur in comparison, confirming that it is the same Lee Chan who was in the other movies. 

The studio ultimately recast Charlie Chan in order to continue the series, but initially replaced Luke with another actor playing Charlie's "Number Two Son"; however Luke did later reprise the role in two of the later movies in the series.

Tell Your Children/Reefer Madness (1938 version) has a newspaper with headlines placing it in the continuity of the second Dick Tracy serial:
http://www.reefermadnessmuseum.org/film/FMfilm/ReeferMadnessFilmA.htm
 

This one is a bit unusual in that the Mission: Impossible characters don't actually meet the Electric Company characters but it is a crossover of sorts between the two shows.

zuckyd1 wrote:

From Grant Morrison's introduction to DK's The DC Book:
"With a storytelling canvas so rich and wide-ranging, and characters so much larger than life that it takes an Omniverse, a Metaverse, to contain their activities, The DC Book provides an invaluable resource and a comprehensive overview, mapping and cataloguing eight decades of worldbuilding into one convenient gazetteer. If this is your first trip to the DC Omniverse, prepare to be spellbound."
That last sentence is also quoted on the back cover.

Sigh. Another bit of "evidence" for those who want to argue that DC (and by extension Marvel) has its own Omniverse. Of course, what it really is, is evidence of another person (Grant Morrison) who doesn't understand the terminology and is misusing it. No more proof that DC has a separate Omniverse from everyone else than the Millennium Falcon "doing the Kessel Run in less than twelve parsecs" is evidence that parsecs are now a unit of time rather than distance.

NINJA HIGH SCHOOL'S INDIE WARS (Antarctic Press, 2020) features crossovers with the following characters/series:

G̵e̵o̵f̵f̵ ̵J̵o̵h̵n̵s̵ Rip Hunter doesn't understand what the Omniverse is.

zuckyd1 wrote:

G̵e̵o̵f̵f̵ ̵J̵o̵h̵n̵s̵ Rip Hunter doesn't understand what the Omniverse is.

He's far from alone on this. I'm really not sure how hard it is to understand - by its very definition, OMNIverse means it includes EVERYTHING. You can't locate something that is outside it, because any new location outside what was previously considered to be the entire Omniverse merely expands the "considered" part - you can find something outside the "known" Omniverse, but only because you've expanded the "known" aspect, not the Omniverse itself.

Last Week Tonight With John Oliver episode from last night featured Reddy Kilowatt. What makes this relevant from an Omniverse perspective is that at the end of the episode, despite the show being non-fiction,

the fictional Reddy killed John Oliver. Presumably he'll be brought back from the dead by next episode.

The newly released Chip 'N Dale: Rescue Rangers movie features a bunch of cameos that effectively crosses over multiple properties, some previously connected but many not. Since it is so new, I'll put the details in spoiler tags.

As you'd expect, there are many Disney properties that have previously crossed over in comics, animated features and the likes of Who Framed Roger Rabbit. Indeed, Roger himself appears. I'm not going to list all of them, with so many prior connections, but ones that haven't crossed over before or only done so very occasionally include: The Incredibles (Frozone and Syndrome both get indirect cameos), Muppets sharing the world with humans and toons (both previously unseen Muppets but also Gonzo), Tigra from the Avengers: United They Stand cartoon, and The Simpsons. Additionally, non-Disney owned properties also show up from a surprisingly wide range of sources - we get appearances from or references to Batman, E.T., Foghorn Leghorn, Yogi Bear, Street Fighter's Chun Li, Samurai Jack, the TMNT, He-Man and Skeletor, "Ugly" Sonic the Hedgehog, the Flintstones, the Smurfs, MC Skat Cat from the Paula Abdul music video Opposites Attract, two of the hideous Cats from the recent CGI movie version of the Andrew Lloyd Weber musical, South Park, Big Mouth, Naruto, My Little Pony, Ice Age, Kung Fu Panda's Mantis, Monster Inc.'s B.O.B., Ren and Stimpy, Rick and Morty, Beavis and Butthead, Doug, Johnny Bravo, Squidward from Spongebob, Pink Panther, Pikachu from Pokemon, Rocky and Bullwinkle, and many more I've forgotten or missed.

CRIMINAL MACABRE/COUNT CROWLEY: FROM THE PIT THEY CAME #1 (Dark Horse, Oct 2022)