The Cheat of Homestar Runner stars in a parody of G.I. Joe called Cheat Commandos. Also, another character, Stinkoman, has his own anime series.
The Strong Bad E-Mails is also a type of show, with Strong Bad giving it a changing opening theme and referring to it as his "E-Mail Show."
Homestar Runner also has Teen Girl Squad, an "indie comic" about four "teenage girls between the ages of 13 and 19" dying in humorous ways, crudely drawn and voiced by Strong Bad.
4 Gregs is a comic drawn by the titular squad. Who in turn were in a comic drawn by the Gregs. Who again were drawn by the girls. And so on. This gets so meta that even Strong Bad loses track.
Dangeresque is a series of Stylistic Suck-laden home-made action movies made by Strong Bad and his friends.
Homestar does a guest appearance on Caleb Rentpayer (which otherwise falls under the "Characters are fans" heading).
Episode 77 of Bonus Stage has Joel and Phil optioned by G4 to create their own cartoon for the network. The show is unfortunately ruined by Phil's endless suggestions. Much of the episode is a Take That! to G4.
Two back-to-back episodes of GEOWeasel feature shows starring the main characters. The first features The Nar Show, which Nar wins in a contest (the flier for which says, "millions will enter, 2.5 will win") and Weas tries at every turn to stop. The second is a dating game that is meant to introduce only one female character to the show.
The Most Popular Girls in School: In Episodes 19 and 20, the high school characters star in a reality show called "Babes Having Babies".
Red vs. Blue: The Sarge movie trilogy, produced by Red Team until the final installment. Only trailers for said movies are shown, but given that it's Sarge writing the scripts, they're predictably centered around him being a grizzled action hero and the Blues being the antagonists... save for the second movie, which managed to be overdosed with different genres.
StarStarSpace in Japanoschlampen, both actually produced by coldmirror. Not only are Wake, Chao and other characters big StarStarSpace fans, but it turns out that Frau Lehrerin is the actress behind Captain Captenne Capitanöse.
Characters are fans
In the Flash series Banana-nana-Ninja!, "Kikyotushi: The Suburban Ninja" is a live-action show about a guy who dresses like a ninja and kills people on the street in Candid Camera Prank fashion.
Smosh Babies has "The LePrawn James Show", featuring a prawn caricature of LeBron James.
A few cartoons mention a comic Soap Within a Show called Caleb Rentpayer, which involves the Zany Schemes of the title character and his long-suffering manservant Tucksworth. It's never shown, only heard, except for a few seconds when Homestar guest-stars (and is the only one onscreen).
Tucksworth: You can't marry your guitar, Caleb! Caleb: I can try.
In the Strong Bad Email "best thing", Strong Bad claims the best thing he's ever seen, done, or eaten is Limozeen: But They're In Space!, a One-Episode WonderBand Toon about his favorite hair metal band, Limozeen.
"Happy Dethemberween" opens with the gang gathered around Strong Bad's television, watching a pastiche of Rankin-Bass stop-motion animated specials called Brundo the Decemberween Yak and the Sword of St. Olaf.
Strong Sad: Can someone please explain again what a mystical sword has to do with Decemberween? Homestar: You hear that nasty ol' puppet! The magic of Decemberween is inside all of us!
is a comic drawn by the titular squad. Who in turn were in a comic drawn by the Gregs. Who again were drawn by the girls. And so on. This gets so meta that even Strong Bad loses track.