The "Night of Elizabeth Taylor", broadcast on CBS around 1995-96, saw a diamond necklace lost by Elizabeth Taylor became a common plot element linking four SitComs — The Nanny, Cant Hurry Love, Murphy Brown and High Society — in one massive crossover. It was intended as an embedded advertisement for Taylor's new perfume, Black Pearls.
Disney did a triple-episode MMC with three of its shows. The show was entitled That's SoSuite Life OfHannah Montana, with one classed as a Suite Life episode, one as a Raven episode, and one as a Hannah Montana episode, where Hannah and Raven visited the hotel the twins live in.
Likewise, there's the Wish Gone Amiss triple-episode, except it is more loosely tied together. It all involves the title characters from Cory in the House, The Suite Life of Zack and Cody, and Hannah Montana making a wish on apparently the same shooting star. Each episode has its own method of returning to the Status Quo — Cory gets a literal Reset Button, Zack and Cody's wish was all just Zack's dream, and Hannah returns her life to normal when Jackson unwittingly wishes that the world did not know Hannah's double life.
There's also the WizardsOn Deck with Hannah Montana that goes by the same formula that the That's So Suite Life of Hannah Montanna did, except with the stars visiting the hotel's ship instead of the hotel itself.
October 2 and 4, 2015 brought us an odd one for Halloween. Literally every Disney Channel show crossed over with each other, among them Girl Meets World, the spinoff of Boy Meets World, finally linking the DCLAU with the TGIF universe and potentially tying it into the Tommy Westphall theory (see below).
Kamen Rider Decade is this in regards to the Heisei era Kamen Rider shows. The main character dimension jumps into alternate universes based on the 9 previous Kamen Rider series of the last 10 years (as well as the canonical universe of Samurai Sentai Shinkenger). Plus, the first movie features every main Rider created before Decade, even ones that only had appeared in one-shot movies previously.
The second movie, a Grand Finale, even includes some postmodern commentary on the "interesting effects on the fiction chosen" mentionned in the opening paragraphs. To quote the original universe Wataru: "The tales of the Riders were something that would be eventually lost to time. But because of Decade's battles, they will remain fresh in people's minds..."
Similarly to Kamen Rider Decade, Kaizoku Sentai Gokaiger is a crossover series. But compared to Decade, in which the main character could only be Riders of the last decade, the Gokaigers can transform into any of the Sentai from the past 34 teams (barring Sixth Rangers - that is, until their own Sixth Ranger joined up). Additionally, Gokaiger establishes that all Super Sentai took place in a single universe.
It wasn't the first time - in fact, quite a few crossovers spanned the entire TGIF lineup from time to time, and even an occasional though less involving tie-in for all ABC weekly sitcoms during this period.
There was also one that was linked by Steve Urkel. He ends one episode of Family Matters by blasting through the Winslow's roof in a jet pack, and he crashes into the Lambert's roof at the beginning of the an episode of Step by Step where the plot centers around him. I forget if any other shows were connected as well.
In Doctor Who, the climax of the Second Doctor serial The Mind Robber. D'Artagnan vs. Cyrano de Bergerac! Blackbeard vs. Sir Lancelot! Plus Gulliver and Rapunzel on the sidelines.
Law & Order crossed over with Homicide: Life on the Street a few times, until they eventually just decided they were set in the same continuity altogether, to the point of having John Munch, who originated in the latter show, permanently set up shop in the former.
John Munch is a central figure in this Hypothesis.
The Earth Day Special, which aired on ABC in 1990, was a huge crossover featuring just about every pop culture icon from The '80s in a very bizarre, thoroughly nonsensical plot.
Also in 1990, Cartoon All-Stars to the Rescue, the ultimate Very Special Episode combining well over a dozen Eighties cartoon characters. Aired once and only once, it's full of Narm yet also bizarrely entertaining. Rumor has it that it's never been aired since because Jim Davis claimed he hadn't authorized Garfield's inclusion in the show, though this has since been debunked by Mark Evanier.
In the CSI-verse, the original series crossed over with the Miami series in 2002 to serve as the pilot for the latter; the Miami series then crossed over with the New York series in 2004 to serve as that series' pilot, and the series crossed over again in a two-hour storyline in 2005. The original series also crossed over with Without a Trace in 2007 a two-hour storyline across both series. Following the departure of William Petersen (who opposed the spinoffs and did not appear in any scenes featuring the Miami team in "Cross Jurisdictions"), CBS put together a massive crossover in 2009 spanning all three series that involved the Las Vegas series' Raymond Langston going to Miami and New York while investigating a human trafficking organisation.
The second western MMC took place a year after the first on what seemed to be an ordinary episode of Maverick called "Hadley's Hunters"; during the course of that hour he ran into people from 5 other shows: Lawman, Cheyenne, Bronco, Sugarfoot, and he stops by the office from Colt 45 but nobody was home (a reference to the show being recently canceled).
Strangely he also ran into the parking lot attendant from 77SunsetStrip which was set in the 1960s — I guess he had an ancestor who lived in the old west.