In the TGWTG Team Brawl special, the Nostalgia Critic and Chester A. Bum are both in the fight, and then "Ask That Guy" comes in near the end and gives the entire brawling mass a lecture about misdirected creative energies, and proceeds to take a group photo.
And this was also lampshaded when the three characters all explained how they ended up shaving their beards at the same time.
Don't forget the April Fool's gag where the three characters rotated sets for a video, so that the Nostalgia Critic became severely traumatized by all these horrifying questions he was getting from the audience while That Guy rambled on incoherently (in his same old monotone) and then demanded change, and Chester reviewed an old movie.
In an exclusive video on the Volume 2 DVD, Ask That Guy is asked why he, the Nostalgia Critic, Chester A. Bum, Dominic, Black Dog Bill, and General Zod all look alike. Rather than give a "wacky in-universe" explanation, Ask That Guy just breaks the fourth wall. "Because we're all the same actor, you idiot!"
Lampshaded in Obscurus Lupa's Charmed reviews. One storyline involves Phoebe dating a character whose actor is also on Sex and the City. As a gag, at one point she holds a DVD box for Sex and the City and compares it to her own life. Lupa then questions why she doesn't notice that one of the show's characters is identical to her boyfriend.
New York Magician: Discussed when Michel meets Cthulhu. He's sitting in a bar later on, more than a little disturbed, and points out to the djinn he's discussing the incident with that said Elder is supposed to be fictional. The djinn's response is basically "That doesn't mean he isn't real."
A variation of this trope is often run into in Journal Roleplay games that use "fandom" characters. Many games have a "no fourth-walling" rule—i.e., one character can't reveal to another character that they're fictional, or use knowledge of their original canon to their advantage.
On the other hand, there are pan-fandom roleplays that are more lax about the Fourth Wall, usually with the bottom line being "but only if the player of a character is okay with it". (However, even the most meta-aware No Fourth Wall character is still not allowed to recognize that they're in a Journal Roleplay regardless of premise or setting.)
Slacktivist has a post about the problems that this trope causes for would-be prophets, here.
Both averted and not in The Lizzie Bennet Diaries. The vlog that Lizzie does is available in-Universe and is clearly relatively successful. Most characters are aware of it. On the other hand, none of the characters has ever heard of Pride and Prejudice.
Noob plays with the trope both in terms of the series itself and stuff related to it. Noob-related objects can bee seen in Fantöm's room and the channel that airs the webseries on TV exists. Omega Zell has a ringtone that is none other than his own character song and a shot of the band that performed it is briefly seen between two TV ads related to the Fictional Video Game of the setting. The lead singer of the (real) band is the actor playing Master Zen, a character that is a hiding escaped prisonner.
In the third episode of Amazon's Betas, a background character in a karaoke bar sings "Gives You Hell" by the All-American Rejects. That band's lead singer Tyson Ritter has a recurring role, not as himself but as a drug dealer; in that episode's B-plot, the other characters bail him out.