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 Vince Guaraldi named the piece that would be permanently affiliated with the Peanuts characters through the specials "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 "Silence of the Clamps" the crew finds a robot who claims that his name is Billy West. Fry calls it a "stupid, made-up name".
In "Lrrreconcilable Ndndifferences", a 3010 SDCC panel with Matt Groening previewing "Futurella" set in the 40th century, until immediately cancelled by FOX.
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 writer 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.
In What's New, Scooby-Doo?, JC Chavez 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."
A hair salon in one episode is named "Hairy Shearer's."
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.
In The Critic, Jay happens to be a fan of Jon Lovitz (who happens to provide the voice of Jay).
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 creature of Futurama. Truly parodoxical since The Simpsons is a fictional show in Futurama's universe.
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)).
Additionally, John Witherspoon actually plays Robert in an in-series music video, "Eff Granddad." Robert's comment: "That's the dude from Friday!" Robert also mentions Friday 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.
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, Meg, Lois, and Chris all agree that Seth MacFarlane is a douchebag, which Peter, Brian, and Stewie (all three voiced by MacFarlane) take issue with.
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.
In a brief gag for the Fairly OddParentsNon-Serial Movie "Channel Chasers", Butch Hartmann (the creator of the series) appears in the credits for "Adolescent Genetically Modified Karate Cows".
Parodied in Phineas and Ferb when they make a movie to make Candace famous. Through some careful editing, they make Candace look like Ashley Tisdale, who is Candace's voice actor.