Lucky Star has quite a bit of this thanks to its numerous modern-day media references to anime, manga, video games, etc. Prime examples include multiple shoutouts to its fellow Kyoto Animation anime The Melancholy of Haruhi Suzumiya and the guest appearances of the character's own voice actors
Probably the prime example of this is in the last episode. Patty bribes Konata (voiced by Aya Hirano) with a ticket to a live event where she can meet... Aya Hirano
Of course there's also an earlier episode where Konata attends the Suzumiya Haruhi no Gekisou live concert and sees Hirano on stage.
Aya also appears in the first strip of volume 5 of the manga — eating a choco-cornet, from the pointy end, no less — and is noticed by Konata (and Hiyori, who calls her "that girl from Haruhi)".
During one of the Lucky Channel spots, Anime Tenchou questions Akira (Hiromi Konno) on her feelings for a handful of her previous roles, for her to nonchalantly ask who those people are.
The show also bizarrely folds in on itself in one of the later episodes where Konata, in Ripped from the Headlines fashion, finds a plaque that reads "Konata is my wife."note It could also be a pun on Konata's name, since "konata" means "here" in Japanese. "Here is my wife."
In that same episode, while on the school trip, Konata insists a visit to the local anime studio is a must (guess which one).
A case in Detective Conan once revolved around the eponymous character and his friends meeting singer Minami Takayama. Minami happens to do the voice of Conan Edogawa, and their similar voices were pointed out by other characters. That story also appeared in the original manga. Not so weird, considering mangaka Gosho Aoyama was dating, and then was briefly married to, Minami.
This trope also creates some pretty big Fridge Logic thanks to the crossover film Lupin III vs. Detective Conan. The crossover indicates that both characters exist in the same fictional universe. But that then raises questions concerning the iconic character Sherlock Holmes: in Conan's series, Holmes is a fictional character like in ours, and the original stories are what inspired Shinichi/Conan to become a detective in the first place. In Lupin's series, however, Holmes was a real life person, and an episode of the second Lupin series even has the master thief meeting and impersonating the detective's grandson!
Furthermore, movie 11 establishes that Sato-san is a fan of Lupin III and is therefore upset when a criminal wears a Lupin III mask. Given that she is a police officer and would not be a fan of a real thief, this would only make sense if Lupin III is a fictional character.
An episode of Lupin III (Red Jacket) has an Arab leader learning about the Lupin gang by watching their show on television.
An episode of Akahori Gedou Hour Lovege has two of the Hokke sisters meeting their own voice actresses and then proceeding to argue about which one of them is better.
Completely pulled off in Nogizaka Haruka no Himitsu, Meido Nanami Nanashiro distracts the proud Otaku Nobunaga Asakura by telling him that voice actress Kana Ueda is nearby and proceeds to mimic her voice to make Nobunaga 'chase' the voice actress Of course, Kana Ueda herself is the voice actress of Nanami.
In the first episode of the second season, Yuuto and Haruka (who's voiced by Mamiko Noto), go to see the voice actress of a Show Within a Show. The character she voices heavily resembles Haruka and is also voiced by Noto — in the show's universe. Essentially resulting in Haruka going to meet her own voice actress.
It comes around full circle in episode 10 where Yuuto visits a recording session for Nocturne. It is heavily implied that the five VAs who were recording roles in Nocturne were Mamiko Noto, Mai Goto, Kaori Shimizu, Kana Ueda, and Rina Sato. Also, Asakura tells Yuuto about a certain Reiko Takagi, who is in fact, Asakura's own VA. Taken to Up to Eleven in the Nogizaka Vocabulary Essentials section, where the word is "Wataru Hatano," Yuuto's voice actor.
Done several times in the original Astro Boy. Once, when showing a theme park full of fictional creatures, a villain disparagingly refers to "Those creepy gourd creatures Tezuka draws," indicating that Osamu Tezuka, the creator of Astro Boy, does exist in their universe. Another time, a bad guy taunts the Professor by saying, "And what have you done with Astro? You've just made a stupid show of him for TV!"
This started off in the Light Novel with reference to Yukari Tamura being mentioned as the voice of one of the animals. When she got the actual role the studio asked her to 'sound like Yukari Tamura' when she pointed out she WAS Yukari Tamura, they said 'yeah but try not to sound like one of your characters but as yourself'. Pretty meta.
Bakuman。, which as a manga about trying to create manga is already up to its ass in Post-Modernism, has references to Death Note and to "the author of Death Note" in the very first chapter, although the latter is not referred to by name. Bakuman, of course, is created by the same writer/artist duo that created Death Note.
In The Kindaichi Case Files, Hajime laments that he's no match for Tsuyoshi Domoto who was starring in a movie with Reika. Tsuyoshi Domoto was cast as Hajime in the live action version.
In chapter 107 of The Wallflower, a character mentions the actor who played Kyōhei in the live-action TV series. The best part is that instead of mentioning his name, she says that he played Kyōhei on TV, directly referencing the TV series based on the manga they exist in.
Gundam Build Fighters has an extremely odd version of this thanks to Mr. Ral, a character who looks and sounds exactly like Ramba Ral from the original Mobile Suit Gundam. Unlike most examples of this trope, however, the Gundam anime does exist In-Universe, and most of the characters are huge fans of it — which makes it even more surreal that nobody comments on or even seems to notice the resemblance.
In Re:CREATORS, a normal citizen is shown carrying a bag with the Aldnoah.Zero logo on it. Studio TROYCA is the group behind both series, though the lack of a Re:CREATORS existing there can be excused since a good chunk of the action happens around Fall 2016 rather than Winter, when the show was announced, or Spring 2017, when the show actually aired. Sora Amamiya and Mikako Komatsu also appear in an in-universe press conference thanks to voicing their respective characters (Rui and Celestia) in their own series. This one is explained away as the characters gaining their voices in the real world world from the actors that portrayed them in adaptations.
DEVILMAN crybaby reveals at one point that the original 1972 Devilman anime exists in the universe of the story. One can only assume it's In-Name-Only (which, to be fair, a good number of Devilman adaptations have been, including the 1972 anime which was considerably Lighter and Softer than the manga).
The series makes several references to elements from Rurouni Kenshin, which prominently features characters based off the Shinsengumi (a group also prominently featured in Gintama) in a dour and much less futuristic Meiji era.
Seiyuu example: One chapter has Shinpachi playing Granblue Fantasy. Several of the voice actorsnote twothirds of the main trio voice story characters, for starters have played characters in the game before.