While on the run from the Baron in Girl Genius Agatha joins a circus which is also a traveling Heterodyne show and ends up playing the exaggerated version of her mother the plays feature on multiple occasions.
The interlude City Face is implied by The Rant and the Shout Box (which portray the City Face characters as Animated Actors) to be a fictional film. Its only connection to the Gunnerverse (apart from Word of God saying that City Face is canon) is that the actors include Zimmy, Gamma and City Face, the pigeon Kat rescued.
In the beginning, Ansem Retort was mostly focused on its status as a webcomic about a reality show filled with jerkasses and cloudcuckoolanders who were all addicted to at least 3 illegal drugs each. Cerebus Syndrome has kicked in a bit as the series goes on and things get more serious (well, as serious as that freak show gets, anyway), but they still bring up things like ratings, and in fact often let events occur because they wouldn't have a plot for the next season otherwise.
Nip and Tuck has "Purloined Letters Productions", a shoestring B-movie/direct-to-video company that has, to date, produced such epic hits as Man on the Border, Rebel Cry, 10,000 Ninjas, Gravedigger, and Scurvy Dogs: The Curse of Blackbeard's Treasure.
A Tale Of Fiction features "Bison and the Boar Boys", a comic with no bearing at all to the main plot (as of yet) and no connection to the characters, but which can nevertheless be accessed from one of the characters' laptops ("dragbook"). Also available on the dragbook is an in-universe blog called "I Am Not Fiction". Only Bison and the Boar Boys is regularly updated as of now, although I Am Not Fiction is likely to soon follow suit.
1/0 had "Max's World", a short comic created by Max to illustrate to Marcus what a comic strip is. Since the strip itself (1/0) is about metaphysical issues and the nature of reality, the comic strip characters creating a comic strip may also be an example of Recursive Reality. "Max's World" doesn't last long, though, since the 1/0 characters get jealous of the girlfriend Max drew for his main character and squash him with an anvil in the next panel.
Zombie Ranch is not only the name of the comic but the name of the in-universe Reality TV Show being filmed about the characters' lives. The comic occasionally cuts between the "live" recordings and what seem to be more produced segments being aired sometime in the future.
In Faux Pas, Myrtle, Shipper on Deck, retells Tarzan with Randy as the ape-fox and Cindy as the intrepid archeologist.
In Arthur, King of Time and Space, Arthur and Merlin both have webcomics (Merlin's is later taken over by Nimue). Arthur's is a Life Embellished strip called The Thing of Shapes to Come and Merlin's is about aliens and called The Land of Whant. There was a crossover, in which the aliens came to Earth and befriended Arthur.
In Kevin & Kell, Rudy creates two webcomics. The first is about human versions of himself and Fiona called Rhondo and Viola and the second is about a sheep and called Mad About Ewe.
Billy Thatcher of Morph E is the star of a reality TV show named Pawnography. The show is presently on hiatus due to its star being kidnapped by a flirtatious young mage with blonde curls and an affinity for chess playing reality TV show stars. One of the characters is only aware of Thatcher and his show due an in-universe 4Chan meme using sound-bytes from the show.
In Cucumber Quest: The TV Show Punisher Pumice. Little is shown about it, but we know that Almond is a fangirl. Almond also says in a Q&A that she learned sword fighting by watching TV, "especially from Punisher Pumice" .
In the webcomic El Goonish Shive, some characters are fans of the Lucky Bunny Bounty Show (which was canceled after only 22 episodes).
Girly features Action Up The Butt, which is what happens when you take the concept of Highlander (where killing someone gives you their power), make all the characters 19th century authors (with Sir Walter Scott as the main character), then give them all guns. It was cancelled after the second season.
Mark Twain: So, you've heard those rumours about me, have you? Sir Walter Scott: I... I didn't want to believe them... but... Mark Twain: That's always been your problem, Sir Walter Scott. You never wanted to believe anything. And now, there will be rumours about you. Specifically, your death. And they will NOT be exaggerated!
In Heart Core, it is shown that Ame is really into the cartoon show called Royal Jelly Complex. Most of it has to do with that she is easily distracted by video entertainment.
My Life At War: Vulture is a fan of a show about a mercenary mecha pilot called The Southpaw.
In Dumbing of Age, Walky and Dorothy are both fans of the cartoon "Dexter the Head Alien and the Monkey Master". Both the Head Alien and Monkey Master are characters of another comic by the same author. Carla is a fan of the Ultra Car cartoon, Ultra Car's gynoid chassis in Shortpacked! being the genesis of Carla.
In Rain, the titular character is a big fan of the manga "Black Wings Kaminari," and is more than happy to get her friends hooked on it as well. Chapter 25 is a long dream sequence where she envisions herself as the character.
Leif & Thorn features a bunch, mostly sequences from from Hedge & Grassie, a sitcom Thorn is a fan of. Other referenced properties include Leachtric (musical), The Holger Saga (literature), Pretty Combat Mountaineer Geirskögul and Magical Storm Rune Svanhvit (magical-girl comics).
Multiple Girl Genius characters are fans of the Heterodyne stories and plays, which are based on Agatha's parents and uncle even if a lot of the material is made up whole sale or incredibly otherwise off when compared to the reality of their lives.