A neighbor who drops in on a regular basis and mooches around. He or she is often a friend of one of the household kids who is either very annoying or very cute. Because he or she is the neighbors' kid and we never meet his or her parents, he can exhibit all sorts of zany behavior without the need for too much explanation.
In most cases, the main characters have given up on trying to get the drop-in characters to knock, a la Kramer from Seinfeld, or Lenny and Squiggy on Laverne and Shirley. Considering how much of a pain in the ass this sort of drop-in tends to be, the viewer is sure to wonder whether the host has ever thought to just lock the door, but don't expect that to be addressed in the show. It's exceptionally rare to even see a lampshade hung here.
Sometimes the drop-in character's arrival is presaged by an ironic foreshadowing — see Inadvertent Entrance Cue.
Compare Your Door Was Open.
Not Truth in Television; trying this in Real Life is a good way to get the police summoned, at least if one is a teen or older.
For a different sort of "drop in character", see Face-Heel Turn or Moral Event Horizon. For yet another different sort, see Suspiciously Similar Substitute.
Miu from Strawberry Marshmallow drops in by Nobue and Chika by jumping the roofs of their adjacent houses and climbing in through Nobue's bedroom window. She even does this in the middle of the night to sit on Chika's stomach, for no other reason than to be the Jerkass she always is. Heck, she even drops in through the window when the crew are staying at a hotel, purely out of force of habit.
Yotsuba&! is the extremely cute example of this trope. She doesn't distinguish much between her own house and her neighbor's house and even calls the next door neighbor mom "Mom" just like her own kids do.
Lucy from Fairy Tail is an inversion; she's the one that gets dropped in on... constantly... by everyone. People make themselves so comfortable at her house it's ridiculous.
Ikkyu, Nozomu's "one day friend" in Sayonara, Zetsubou-Sensei sometimes plays this role, with the twist that he doesn't drop in at Nozomu's home, but rather at the school Nozomu nominally teaches at, which makes him a more bizarre example, as does the fact that he isn't really a close friend of the protagonist. Lampshaded once by Nozomu, who asks him "Why are you here? What is it you do at this school again?"
Almost every major character has played this role at least once. Many a chapter or episode has started either with Itoshiki at home when his students suddenly barge in, or one of his students (usually Nami) at home when Itoshiki, who just happens to be passing by, overhears them saying something that causes him to barge in and launch into one of his rants. The biggest exception is Matoi, who doesn't drop-in because she was already there all along.
Most of the other tenants of the titular Maison Ikkoku force there way into Godai's small apartment room when they feel up for a bit of a drunken party. He does try locking people out, but it doesn't work very well, particularly Yotsuya-san who just breaks holes in Godai's wall when he does (so he can cross the room and peep at Akemi next door through a peep hole in Godai's closet).
The Golden Girls: Stanley Zbornak, ex-husband of lead character Dorothy Zbornak (Bea Arthur), often dropped by — usually unannounced — to try to win Dorothy back.
In Friends, every character does this at one time or another, usually at least once an episode.
Slightly lampshaded at one point near the end of the series when Monica can't remember the last time her apartment door was locked.
Also lampshaded when Ross is about to finally tell Rachel he loves her, but is interrupted by ex-fiancee Barry bursting in the door and beating him to the punch.
Ross: WE HAVE GOT TO START LOCKING THAT DOOR!
And again in the last episode, where Monica and Chandler, about to leave their apartment for good, solemnly drop their keys in a dish for the landlord to pick up - followed by all the other leads adding their own key to the dish.
This is also mentioned by Monica, in episode 3.19, when she enters her house only to find Phoebe, Chandler and Ross there.
Monica: Hello, people who do not live here! I gave you a key for emergencies.
Phoebe: We were out of Doritos.
Howard on The Bob Newhart Show was a classic Drop In Character, often having just flown in or about to fly out.
Nick at Nite once ran a campaign including multiple clips of Howard asking "what's for dinner?"
On Newhart, this role was filled by Larry, Darryl, and Darryl.
Also on Happy Days: Potsie, Ralph Malph, Lori Beth, Arnold, Al Delvecchio, Jenny Picalo, Roger Phillips, Melvin Belvin, Pinky Tuscadero, Officer Kirk...pretty much every character on the show who wasn't a member of the Cunningham family wound up crossing their threshold uninvited at least once.
Spoofed in at least one of Cracked magazine's "Happy Daze" parody comics, as illustrated above.
As already mentioned, Kramer on Seinfeld is one of the most popular, well-known examples. A little more justified than other instances in that they lived in an apartment building with security so Jerry only had to worry about other tenants dropping in unannounced.
Technically George and Elaine are this too, they just don't fit the conventions of the trope nearly as well.
They eventually had to cut down on his drop-ins because the studio audience would applaud for like a year when he entered and throw off the timing of the scene.
In one episode, Jerry actually had the door locked for an unrelated reason leading to a loud crash as Kramer tried to perform his trademark slide.
Family Ties has several of these — Skippy was the nerdy one, Nick was the punk one (well, TV-punk).
"Bud", "Cockroach" and several of Vanessa's friends on The Cosby Show.
The Australian sitcom Hey Dad..! had several drop in characters over the years, most famously "Nudge". Inexplicably, one of the drop in characters, Ben, ended up moving into the family proper.
Inverted in Keeping Up Appearances, in which the main character, Hyacinth, forcibly drags Elizabeth, her neighbor, into Hyacinth's house and forces tea on her.
Usually in Royal Doulton teacups (...with the hand-painted periwinkles...). The fear of dropping these teacups has turned Elizabeth into an agoraphobic wreck.
Charley Dietz from Empty Nest would frequently drop into Harry's kitchen just to raid his fridge and hit on his daughters.
Hakeem Campbell on Moesha. All of his meals appear to come from the Mitchell household ("My favorite cook!" "Your only cook!").
Lampshaded in the Coupling season 4 episode 'Nightlines' in which one of the characters barges into the home of another (due to her retaining the front door keys of anyone she's ever had a relationship with) which causes another to state that she has to stop doing that because 'this is not, repeat: not an American Sitcom'.
Pretty much the entire teenage cast of That '70s Show with regards to the Foremans' house, particularly the basement (though naturally they have no problem wandering about the rest of the house and enacting other elements of the trope, such as mooching food and pilfering Red's beer). Occasionally lampshaded, such as one time when Kitty mentions an embarrassing secret of Eric's aloud.
Eric: Mom! Be careful. At any given time, there are at least 3 individuals in this house that would use that information to destroy me.
Lampshaded again in one episode where Eric, Hyde, and Laurie are all grounded. Red goes around to perform a "bed check", with Eric having Donna in his room, Hyde having Jackie, and Laurie with Kelso. Then, Red goes down to the basement, where Jackie, Fez, and Kelso are sitting watching TV, like nothing is wrong.
Martin had a character simply referred to as "Brother Man", who occasionally invited himself into Martin's apartment through his window.
Reba featured possibly the worst example of this trope, as Reba's ex-husband and the woman he left her for felt perfectly justified in walking into her home without any form of notice or even knocking. It costs too much effort for Reba to change locks all the time just to keep them out, and plots advance faster without doors locked.
She did lock the door occasionally, but only for gags (though I believe one had Barbra Jean refusing to give up).
In Kenan & Kel, we never see Kel's home or family, while much of the action takes place at Kenan's home and his parents and sister are relatively major characters. Kel also hangs around Kenan's place of work, Rigby's.
In The Sarah Jane Adventures Maria's parents are divorced and she lives with her father; her mum frequently drops in unannouced.
Bernard "Beans" Aranguren, in Even Stevens. He was essentially a parody of the Drop-In Character, since the Stevens family couldn't keep him out of their house no matter what they did. And not only were his parents never seen, but the only episode where anyone tries to track Beans back to his house led two of the main characters to conclude that Beans was in fact an alien.
Sam and Freddie from iCarly, although they're part of the main ¡Three Amigos!, they were once called by Spencer "her two friends who never hang out at their own homes."
Being a recurring side character, Freddie's mom is a classic example of the trope.
At one point Spencer mentions he almost didn't recognize Sam and Freddy since they weren't sitting on his couch.
It should be noted that due to Sam's skills with a lockpick, locking the door on her is pretty much useless.
Monroe Ficus was this in Too Close for Comfort, generally when he appeared, it was obvious that he was going to say or do something that would irritate Henry Rush. Eventually when the show became a first-run syndicated series, he would become a live-in character, when he moved into a third floor addition of the Rush house, after his apartment is torn down. By the time the series was rechristened as The Ted Knight Show for its final season, he reverted back to being a drop-in character when the Rushes moved from San Francisco to nearby Marin County.
Raj, Howard, Bernadette and Amy are usually invited over for a real reason like Vintage Video Game Night. Penny, on the other hand, once came over in her pyjamas muttering "Out of coffee, need coffee" and proceeded to pour herself a cup like it was nothing.
In the early seasons Penny was often lampshaded as taking advantage of Leonard's need for female companionship (even if unintentionally), and in later seasons she was Leonard's girlfriend, and so it made sense for her to be able to pop in whenever (many serious relationships involve each partner giving the other one a copy of their key).
Ed on Northern Exposure. He has a habit of already being in the room before someone even notices he's there, and he's been asked on multiple occasions whether he believes in knocking.
Fred and Ethel Mertz on I Love Lucy, either through the front door, or through the balcony door next to the kitchen.
On Sesame Street, Ernie or Bert can expect any of the lovable monsters to drop in for a chat and an exercise or lesson.
On Wizards of Waverly Place since a lot of the plot takes place in the sub shop, it's easy for characters to 'drop in'. Harper was this until she actually started living with the Russos.
Shawn on Boy Meets World, generally through the back kitchen door or Cory's bedroom window via a tree in the backyard. It's been vaguely implied from time to time that he does so to escape violence and hostility in the trailer park, or because he doesn't have enough food to eat at home. Gradually lessens as he grows up.
Elmo, the neighborhood kid who often drops in on Blondie's Dagwood, usually while he's trying to nap. On at least one occasion this was lampshaded by Dagwood saying, "I've got to put a lock on that front door."
Nermal became this in Garfield after his origin was retconned (he originally belonged to Jon's parents).
An inversion of this, of course, would be Dennis the Menace (US), whose title character frquently dropped in unannounced (and unwelcomed) on crotchety neighbor Mr. Wilson.
Sally Forth has one of these in Faye, the best friend of the Forths' daughter Hillary. She often pops up at odd times, and the strip will lampshade this by having Sally or Ted sarcastically ask whether she's moved in with them.
Get Fuzzy has Bucky's British cousin Mac Manc McManx (or "M3"), who has been squatting in Rob's flat for years, much to Bucky's discontent.
Inverted on Aqua Teen Hunger Force, where the main characters are the ones who are always dropping in at the weird next-door neighbor Carl's house — usually to use his swimming pool without permission. In one episode, he builds a laser grid around his house to keep them out. Naturally, the results are less than optimal...
Ron Stoppable is pretty much this, almost never seen at his own house, he's even there when the Romantic False Lead is hanging out with Kim Possible during The Movie. On the occasions when he is seen at his own house, it usually foreshadows an event of real significance, such as an impending move to Norway or the acquisition of an adopted sister (none of which his parents ever tell him about in advance).
Done frequently in My Little Pony: Friendship Is Magic. Though an Ensemble Cast, whoever is the central character of the episode in question will frequently find themselves badgered by uninvited guests. Aloof Grumpy Bear Twilight Sparkle is naturally the most frequent victim to this, most often from Pinkie Pie (who has a tendency to bring a party load of other Drop In Characters along with her) and Rainbow Dash, who often crashes into someone's house.
Rainbow Dash also barged into Fluttershy's house at night in one episode for the purpose of dragging her out of bed.