Magoo's Puddle Jumper: Magoo, driving a car underwater, thinks all the shipwrecks and litter at the bottom of the ocean are urban decay in Beverly Hills, saying "Look how they let the old Backus mansion run down." Magoo was voiced by character actor Jim Backus.
In one episode of This Is America, Charlie Brown, the Peanuts gang visit the Smithsonian Institution. Among other things, Charlie Brown and Lucy discover the lunar and command modules from Apollo 10 (nicknamed "Snoopy" and "Charlie Brown," respectively), and a Peanuts Sunday strip.
For that matter, the caps that the astronauts wore under their helmets were referred to as "Snoopy Caps" for their resemblance to the aforementioned animated pup. No doubt Snoopy would enjoy it.
In another episode of the series, it is declared that the Vince Guaraldi piece that would be permanently affiliated with the Peanuts characters through the specials was named "Linus and Lucy" "by coincidence."
Jackie Chan Adventures episode "And He Does His Own Stunts" has Jackie and company going to Hollywood. Of course, the actor Jackie Chan doesn't exist and this is confirmed by people asking "Who's Jackie Chan?" Furthermore, once Jackie is found by a studio and Hilarity Ensues, a hot-shot director claims that there will never be a Jackie Chan in Hollywood.
In the Futurama episode "A Head in the Polls," Leela, Fry, and Bender visit the Head Museum, where the preserved, living heads of various famous people are kept. The head of Katey Sagal (Leela's voice actor) is briefly shown in the TV stars section.
In "The Honking," Calculon reveals that he is almost a thousand years old, and has had many different identities during his lifetime. "I was all of history's great acting robots: Acting Unit 0.8, Thespo-mat, David Duchovny!" However, in the very first episode of the series, Duchovny's preserved head is on display in the Head Museum.
In the final arc of Spider-Man: The Animated Series, Spidey visits several alternate universes each with their own Spider-Man. In one of those, Spider-Man doesn't exist — but the comics and films do, and its "Spider-Man" is an unnamed actor who plays him. Spidey then meets Stan Lee and talks to him until Madame Webb arrives to take Spidey back. When he sees Webb, Lee remarks "Who is that exotic woman?". Madame Webb was voiced by Joan Lee, Stan Lee's wife.
In one Darkwing Duck episode, Darkwing tries to sell a screenplay about himself, but the studio rejects him. He threatens: "I think Disney will be more forthcoming!" To clarify, Darkwing was actually trying to sell the script of a licensed comic book about himself, as he decided to personally write the script after seeing the initial treatment negatively portraying him as a coward. However, to get him back, the head of the company refuses to publish the comic, resulting in said Disney line. Interestingly enough, at the time the episode aired Disney actually did have their own temporary comic company, and one of the titles they published was a mini-series that adapted the show's 2-part pilot.
An in-universe example is how some episodes of the series show famous fictional characters as both fictional and real people interacting with the other South Park characters. For example Mickey Mouse ("The Ring"), Freddy Krueger ("Insheeption") and Dick Dastardly ("Handicar") are actual people in said episodes, but also appear among the fictional characters in the "Imaginationland" trilogy. Jesus is also both a South Park citizen and a fictional character in Imaginationland together with other religious figures. Best not to think about it too much!
In What's New, Scooby-Doo?, JC Chasez hatches a plot to frame Mystery Inc by hiring some movie extras to impersonate them (he impersonated Scooby). When Mystery Inc discover this, Daphne is disappointed that she was played by an extra, saying 'What, was Sarah Michelle Gellar busy?'
The impersonators of the gang were caricatures of their voice artists—Frank Welker, Mindy Cohn, Grey Delisle, and Casey Kasem.
An Animaniacs spoof of Beauty and the Beast cast the Tasmanian Devil as Beast. In a gag similar to the Kareem Abdul-Jabbar scene in Airplane!, the Warners recognize him and ask him to do "that thing he does," besides Beast vehemently insisting that "me not Taz."
The famous season 4 Monorail episode was written by Conan O'Brien. In season 5 Bart appears on Late Night with Conan O'Brien.
The end of the episode "Bart the Murderer" features a sensationalistic made-for-TV movie retelling the episode's events. In it, Fat Tony is said to be played by Joe Mantegna, who provides the voice for the Fat Tony character in the show.
Danny Elfman's jaunty "Simpsons" theme has been heard as diegetic (in-universe) background music on the show. Hilariously, when Simpson family members hear the music, they don't realize it's about them!
Matt Groening is known in The Simpsons as the creator of Futurama. Truly paradoxical since The Simpsons is a fictional show in Futurama's universe, and that Futurama's characters would later appear on the show.
Richard Dean Anderson guest stars in Kiss Kiss, Bang Bangalore as the actor starring in Stargate SG-1. In that series, Anderson's character - Jack O'Neill - is a fan of The Simpsons.
In "Bart vs. Thanksgiving", a Bart Simpson parade float appears on TV just as Homer looks away.
Another episode has Homer getting barred from Moe's Tavern and having to find a new bar. At one point he wanders into Cheers, the same bar containing the same characters from the show. One of the the characters shown (though not in a speaking role) is Frasier Crane, played on Cheers by Kelsey Grammer, who also plays Sideshow Bob on The Simpsons (this would also seem to indicate that Springfield is somewhere near Boston (though probably not)).
Also, Alec Baldwin. What makes this especially funny is that in the earlier episode Homer had actually worked as an errand boy for Baldwin (and Kim Basinger and Ron Howard too) and had spent a great deal of time with him. The voice of the marine biologist should have been very familiar to him.
Artie Ziff (Jon Lovitz) once visited Moe's. As he walks in, he sees several of the other characters whom Lovitz had voiced throughout the show's history. They all greet him in Lovitz's/Ziff's voice.
Similarly, at the end of Bleeding Gums Murphy's "death" episode, Mufasa, Darth Vader, and James Earl Jones all appear as ghosts in the sky alongside Bleeding Gums. When Murphy tells them to go away, they all say "We're sorry" in the same voice triple-recorded.
"Co-Dependent's Day" introduces the Cosmic Wars franchise, an obvious parody of Star Wars, complete with a blatant Jar-Jar Binks potshot in the form of Jim-Jam Bonks. Trouble is, those movies have already been referenced by name in-series several times before.
Winked at in the episode "...Or Die Trying", which starts off with a trailer for Soul Plane 2: The Blackjacking. Among the numerous celebrities noted to star in it are John Witherspoon and Gary Anthony Williams, the voice actors for Robert Freeman and Uncle Ruckus respectively.
Additionally, John Witherspoon actually plays Robert in an in-series music video, "Eff Granddad". Robert's comment: "That's the nigga from Friday!" Robert also mentionsFriday many other times throughout the series, once referring to it as his favorite movie.
In Family Guy Presents: Laugh It Up, Fuzzball, Darth Vader (Stewie Griffin) and the Emperor (Mr. Pewterschmidt) try to goad Luke Skywalker (Chris Griffin) into joining the Dark Side of the Force by mocking Seth Green, who is Chris Griffin's real-life voice actor. A double example, because Luke Skywalker shouldn't be aware of the existence of Chris Griffin or Seth Green!
Not to mention Mr. Pewterschmidt "playing" the roles of Emperor Palpatine and Uncle Owen, making a fictional character paradox. It's just a shame there weren't any lampshades hung.
Also in the Star Wars specials, Lois portrays Princess Leia. Meanwhile, Mon Mothma is played by Peter's boss Angela, who is voiced by Carrie Fisher, who played Princess Leia in the actual Star Wars films.
Of course, at the end of the first two specials Peter argues that no one watches Robot Chicken (created by/starring Seth Green), which Chris defends. The second time, Peter even brings up the bomb movie Without a Paddle (starring Seth Green). At the end of third one, Chris, Meg (Mila Kunis), and Lois (Alex Borstein) all agree that Seth MacFarlane is a douchebag, which Peter, Brian, and Stewie (all three voiced by MacFarlane) take issue with.
In another episode Brian starts talking about Black Swan, but Stewie cuts him off before he can mention Mila Kunis by name.
Subverted for a throwaway joke in Earthworm Jim. When Professor Monkey-For-A-Head's monkey gains control of their shared body, it takes the opportunity to watch the aforementioned show.
In an episode of Sabrina: The Animated Series, Sabrina ponders "Who needs Melissa Joan Hart's autograph?" Melissa Joan Hart plays both of the aunts in the animated series, and she also played the title character in the earlier live-action series of Sabrina the Teenage Witch.
The Fractured Fairy Tale "Sleeping Beauty" had a Prince Charming that was a caricature of Walt Disney. He used the Sleeping Beauty mythos to create an amusement park around it.
ABC nearly balked at the Beany and Cecil cartoon in 1962 because they thought Dishonest John looked too much like Walt Disney.
Played for laughs on American Dad!. When the CIA develops what is essentially a holodeck, Stan and his coworkers liken it to Star Trek: The Next Generation, but Bullock doesn't get the reference. He then remarks "Some of us spent the late 80's and early 90's getting laid."
A weird Animated Actors example; in the Duck Dodgers episode "Talent Show A-Go-Go", Dodgers (who is credited in the opening credits as being played by Daffy Duck) steals Tom Jones' voice. In the past, Tom Jones gets Dodgers' voice, and complains he sounds like Daffy Duck.
Rebecca Sugar, the creator of Steven Universe, has stated that Garnet's favorite music artist is Estelle, whom Garnet is voiced by. This eventually manifests in-show as a "Stella" album in "Steven's Birthday".