When investigating Mugen Academy in Sailor Moon, the "Inner Senshi" get a Sailor Stack◊, but they get discovered by a security guard and their resulting panic causes them to fall over (except Makoto, who was standing in back, being the tallest of them, and not leaning on anyone else).
In the final episode of Omamori Himari, we get a Scooby Stack featuring the entire Extended Harem. It's so tall, the camera has to pan up to have room for them all, and even then, the Ojou has to walk out into plain view since there's no more room along the doorframe.
The 2000 TNT Original version of Don Quixote did this at one point, with Don, Sancho Panza, and Sancho's donkey all peering around a boulder.
Misfits has one with Curtis, Simon, and Rudy pondering the presence of Seth the Power Broker in the Community Centre.
Buffy the Vampire Slayer used this in the episode "What's My Line, Part II." When Xander and Cordelia are leaving Buffy's house they peek around a corner Scooby-Stack-style. This episode also features the first labeling of the Buffy crew as the "Scooby Gang."
The Doctor, Clara, Angie, and Artie stick their heads out of the TARDIS door in the episode "Nightmare In Silver".
The Second Doctor and his companions while they are peeking out of a London Underground tunnel in "The Web of Fear".
The X-Files: Agents Mulder and Scully look this way into Izzy Berkowitz's room in "Post-modern Prometheus". Mrs Berkowitz is looking directly, standing in front of the door.
Ultraman Mebius episode "An Old Friend's Visit". Complete with anime sound effects.
Often used in promotional shots for sitcoms. Frasier has a promo photo where the cast, including Eddie the wonder-dog, are stacked around the doorframe of Frasier's apartment, with Frasier at the bottom looking rather put-upon. Everybody Loves Raymond used a similar promo photo, with Ray at the bottom trying vainly to shoo them out of his house.
In one episode of How I Met Your Mother, the entire gang are trapped in Barney's apartment and don't wish to be spotted by him, but they peek at him and his date through a door and their stacked heads form the stock pose.
Ally McBeal: When Ally is breaking up with Larry, the entire office spy on her and try to learn the latest gossip or see how she's coping. At one point, they all (minus Richard who is in LA) stick their heads in her door and immediately pull them out when Ally notices them.
The Beatles posed this way several times. The shot used for the picture sleeve◊ for the American "Lady Madonna" single is one example; another can be seen here◊.
Izzy, Eva and Noah perform one in the season one finale while waiting for Justin.
Bridgette and Courtney do it in "Not Quite Famous" from behind a curtain.
Played With in one episode of Sponge Bob Square Pants that focuses on SpongeBob trying to train Gary, his pet snail, for the coming snail race. After unsuccessfully trying to get Gary to practice sprints a couple of times, but Gary immediately goes to watch the television, Squidward leans in a nearby window to taunt Spongebob and boast about his purebred snail's chances of success. Cue Patrick leaning in the window above, boasting about his "snail"(it's actually a rock). Squidward lampshades when he asks what Patrick is standing on, cutting to the outside of the windows to see Patrick actually standing on Squidward's back. When Squidward later leaves, Patrick falls out of the window he's in and then pops back in Squidward's window.
Jay, Cole and Zane do it behind a column in the first episode of Ninja Go: The Way of the Ninja.
In one episode of Super Mario World, Mario and Luigi peer behind a wall to spy on Cheatsy with Yoshi sticking his head vertically in between them.
The Tom and Jerry short "Johann Mouse" has five people popping their heads through a door in order to listen to Tom's piano playing.
Strawberry Shortcake Berry Bitty Adventures has Lemon, Raspberry, Blueberry and Plum all Scooby Stacking in Lemon's doorway doing the "after you, no after you" schtick to avoid being the first to say something they fear will hurt Lemon's feelings.
Legend Of Korra: has one in "The Guide". The adult and child halves of Tenzin's family each peep in on opposite sides of a doorway.
In the episode of Avatar: The Last Airbender called "Sokka's Master", Aang and Katara do this while watching Sokka create his space sword.