Seen It All

A cat chasing a car? They've seen it. People playing card games for the fate of the world? They've seen it. They've explained the Noodle Incident twice. They even know what the Cow Tools are for. Yes, these characters have Seen It All.

They usually come in three types:

Completely Deadpan. They may not even crack a smile because they've heard every joke before. They're usually already this type when they're first introduced, and their experience is implied rather than witnessed. They're not Deadpan Snarkers; they have the deadpan, but their wording is completely serious.
Weary Traveler. We've seen this character go through hell and back, and can understand their sigh of boredom when the most bizarre things happen. This character may be a Deadpan Snarker, and mildly Genre Savvy. It may be a defense mechanism to deal with all the weird crap they keep going through.
Smug Know-it-all. Take the Weary Traveler, but instead of a tired and deadpan reaction to repeat weirdness, they'll smugly say that they knew it would happen, and are disappointed that the event was so predictable. This type is generally reserved for wisecracking heroes (or their sidekicks) or Large Ham type villains who disdain the hero's predictable motivations.

Most Men in Black are type 1. If it is a protagonist, they are usually a type 2. Villains and tricksters tend to type 3.

When the bored residents in an urban center have Seen It All, the trope is City of Weirdos.

Examples of Seen It All include:

Advertising

From The Eighties, this commercial for Keebler Fudge Stripe cookies. A stage magician isn't too impressed by "the old fudge on one side trick", but he takes notice of the part with stripes.

Anime and Manga

Evangeline of Mahou Sensei Negima, an ancient vampire (born circa 1400), fits into type 2: the existence of a time-travelling device impresses her somewhere below the level of a shrug or a nod. She says that overall the ability to travel to the moon, the telephone, and the internet surprised her far more when they first appeared. She occasionally slips into type 3.
Keiichi from Ah! My Goddess. Especially the first movie. A girl kisses him, grows wings, and jumps out the window and flies away and he doesn't even blink.
Kyon, from Haruhi Suzumiya. He even complains in his head that weird stuff is normal to him now.
Sagara Sousuke from Full Metal Panic!, especially in Fumoffu. He and Kaname get stuck in a broken down house that is known for having ghosts. There are a ton of scary, bloody ghosts and corpses that freak Kaname out. For Sousuke, however, none of it fazes him, and he only notices if they happen to have weapons. The reason being, of course, that he's been desensitized to things like that due to all the wars he's been through.
- An equally good explanation may be that he has no idea why it's scary, having no prior experience with horror movies or ghost stories and therefore lacking the social conditioning to react appropriately—all he is looking for is the horrors he's used to (like the telephone-mine).
- Also, ghosts are probably downright comforting when one of your life's less traumatic moments was nearly dying in a plane crash along with your parents when you were three years old.
Maiza Avaro from Baccano!! is a type 2 example, both because of some traumatic experiences and because he's been around long enough to actually have seen it all.
In Durarara!!, Kasuka Heiwajima's deadpan Stoicism never lets up even in the worst or most ridiculous of situations. And oddly enough it doesn't even seem to have been caused by growing up with Shizuo, a Person of Mass Destruction, as a brother, since he had the deadpan expression even when Shizuo first lifted a fridge to try and crush him.
In Puella Magi Madoka Magica, Homura is a type two, due to a Groundhog Day Loop caused by her wish. The reason that she's The Stoic is because she's already experienced the events of the show at least four times already.

Comic Books

One of the funnier works to come out of the Marvel Civil War storyline was the Fantastic Four's Ben Grimm going to Paris. When The Heroes of Paris urgently enlist his aid, he quips in a weary type 2 tone (paraphrased) "What is it? Skrull impostors? A Super Registration Act? A mind controlled clone army?" The French heroes just look at each other in confusion and say no, it's the "Underground Emperor" who wants to collapse Paris by tunneling beneath it. His only response is a teary-eyed "I love Paris".
A lot of modern versions of Batman go this way. Specifically Grant Morrison, Warren Ellis and Kevin Smith.

Batman: Not a murder method I've seen. And I've seen most of them.

Spider-Man is a combination of Type 3 of this and Deadpan Snarker.

Spider-Man: All the crazy wamajama I've seen in my life I am whatever the opposite of a skeptic is.

The citizens of Metropolis and Central City tend to be this. The citizens of Gotham, on the other hand, are much less savvy than would be expected, although the police force falls closer to this trope.
Rick Jones, all the way.
Nick Fury is nearly a hundred years old, has been through three wars (WWII, Korea and Vietnam), been through paratrooper school, demolition trainig, trained with the Army rangers and the Green Berets and worked for the CIA. This was all before he joined S.H.I.E.L.D.

Film

Agent K from Men in Black is the primary example of the type 1 version. He monitors and fights aliens from around the universe; what we consider weird, he considers just another day on the job.
- J turned into a type 2 in the second film.
- Z is also a type 1, possibly even more than K.
Agent Simmons from the Transformers movie. Optimus Prime even noted that they weren't surprised to see the Autobots, they just didn't expect them to show up.
- He does seem to be surprised at one thing in the sequel though; that Wheelie had been managed to have been "tamed" by Mikaela.

Simmons: All my life I've been searching for aliens. And you've got one tied on a leash like a little Chihuahua.

The Curious George movie features a cab driver who claims to have seen it all in New York, even a giant monkey wreaking havoc in the streets. At the end he sees a giant jungle idol in the museum and says, "I haven't seen that before... and now I have."
In a famous scene in the 1980's blockbuster film Crocodile Dundee, the titular character and his love interest are held up at knife point by some New York City thugs wielding switchblades, instead of being frightened and bartering for their lives to be spared, he simply pulls out a Bowie knife and calmly proclaims, "That's not a knife. Now, that's a knife." Then the thugs run away in terror.
The film Groundhog Day uses the Weary Traveler to great effect. Phil Connors finds himself stuck in Punxsutawney, Pennsylvania and reliving Groundhog Day every single time he wakes up. He kills himself multiple times, but each time he wakes up at 6:00 am like nothing happened. Eventually, he decides to use his situation for good and he takes to memorizing the events of the day and getting to know everyone in the town. This eventually leads to him obtaining a vast knowledge of everything that happens and everyone in the town.
By the time Resident Evil: Degeneration rolls around, Leon Kennedy has already spent the course of three separate games fighting every kind of zombie plague you can imagine, and winds up as the Completely Deadpan type of this trope when he gets sent in again during the movie. He rarely even changes facial expressions throughout the movie, let alone indulge in the wisecracking he was known for in Resident Evil 4.
Roxanne in Megamind is the smug type, she's the go-to Damsel in Distress and is rescued on a regular basis so that nothing really bothers her. That said, the events of the film basically go Off the Rails from the norm and everyone gets thrown off.

Literature

There's an alien in The Hitchhiker's Guide to the Galaxy like this: Wowbagger the Infinitely Prolonged from Life, The Universe, and Everything, who was cursed with eternal life. He's watched every single movie in existence thousands of times, and has grown so bored he's resolved to insult every single person in the universe—in alphabetical order.
- There's also a scene in The Restaurant At the End of the Universe where Zaphod tells a Megadodo Publications desk clerk, "Don't you try to outweird me, three-eyes, I get stranger things than you free with my breakfast cereal."
- And of course Arthur Dent himself eventually becomes this.
Detective Inspector Jack Spratt of Jasper Fforde's Nursery Crime series. Years of dealing with crime among Reading's population of Nursery Rhyme characters means that he hardly bats an eyelid over prosecuting the Three Little Pigs for killing the "Big Bad" Wolf, or investigating who killed Humpty Dumpty.
Miracle Max from The Princess Bride, on being given a corpse to revive.
G. K. Chesterton's Father Brown, a little round priest who has seen it all, and somehow forgiven everyone. In his first story, "The Blue Cross," when the great thief Flambeau waylays him, Father Brown has seen it coming, and has already outsmarted Flambeau before the finale. And he has planned out counters for techniques so criminal and horrific that even Flambeau is shocked. The man is Crazy Prepared.
- The man, as he points out himself, sits in the confessional, and listens to people telling him about the horrible things they do. He's not likely to be naive. (Indeed, Chesterton was inspired to create the character after overhearing two Cambridge students commenting on the "innocent and ignorant" life of a priest—who happened to be Chesterton's friend and earlier that day had been talking with him about the horrors of crime.)
Robert E. Howard's Conan has traveled from the frozen wastes of Asgard and Vanaheim to the southern jungles, from beyond the Barachan isles in the west to Vendhya in the east, has been a thief, a mercenary, a pirate, a tribal chieftain (of four different tribes in four different parts of the world) and a general all around adventurer before becoming king by his own hand has fought sorcerors, demons, apemen and giants and discovered lost cities.
Lucy from Someone Elses War has this attitude, along with an air of general defeat, a direct result from having spent her entire life with the Lord's Resistance Army as an unwilling captive.

Live-Action TV

Michael Weston from Burn Notice is type 2. He always knows what to do when things go wrong, he just doesn't know when they'll go wrong.
An argument can be made that Claude Rains, the Invisible man from Heroes, is a type 2 version of this. He's a former Company Man in Black, invisible, and he has alluded to a hobby of randomly following people around, so he's seen a lot.

Claude: "People suck, friend! Never forget that!"

- And:

Claude: "Everyone's like the rest--that's why they're the rest!"

Pretty much everybody in the later seasons of Stargate SG-1, possibly the most Genre Savvy group of heroes this side of the Discworld. At one point, General Hammond believes Daniel's claim that he has intelligence information from a dream he had, and explains his credulity with, "The things I've heard sitting in this chair..." Keep in mind he's talking to someone who's died, Ascended To A Higher Plane of Existence, and then came Back from the Dead, at least twice.
- This trope is nicely lampshaded when the other team members express surprise at Daniel's immediate belief in a teenager who claims to be their teammate Jack, somehow youthened about 30 years in his sleep for no apparent reason, in "Fragile Balance."

Daniel: Stranger things have happened.
Teal'c: Name but one.
Daniel Jackson: Well, there was the time he got really old; the time he became a caveman; the time we all swapped bodies...

And then there's Bon-Chance Louis of the short lived and much regretted 'Tales of the Gold Monkey' who owed his name to having overslept his appointment with the guillotine and dropped little remarks implying he'd been everywhere and done everything.
The reapers of Dead Like Me are, unsurprisingly, not fazed in the least by the deaths they witness every day, and don't even react to the many Necro Non Sequiturs with any more than a deadpanned "damn."
Horatio Caine of CSI: Miami.
Played with on The Upright Citizens Brigade. One episode featured an ongoing thread in which various couples keep looking at a great house for sale, only to be driven mad after looking into the bucket!!. Another plot thread involved a grizzled Defective Detective. When his case eventually brings him to the house he looks in the bucket, looks up at the sky and yells "Don't you think I know that!"
Sometimes happens in later seasons of Star Trek series. When something odd happens, they promptly check for everything odd that's happened before, up to and including parallel universes and time travel. "This is Starfleet. Weird is part of the job."
In Doctor Who the Doctor has spent the last 900+ years of his life travelling anywhere in space and time, and has been to (and saved) possibly billions of planets. He swings between a Type 2 and 3, Depending on the Writer and the incarnation.

Amy: Why am I here?
The Doctor: Because... I can't see it anymore. I'm 907, and after a while you just can't... see it.
Amy: See what?
The Doctor: Anything. I look at a star, and all I see is a big ball of burning gas. And I know how it began and how it will end. And I was probably there both times. And after a while everything is just stuff. And that is the problem: You make all of space and time into your back yard, and what do you have? A back yard! But you, you can see it. And when you see it, I see it.
Amy: And that was the only reason you took me with you?
The Doctor: ...There are worse reasons.

- Of course, Ten and Eleven, at least, turn into Doctor-y balls of Squee when something they actually haven't seen before pops up.
The writers of Supernatural like to lampshade the main characters' casual professionalism every now and then. One example is the fourth season episode "Death takes a Holiday", where the Winchesters discuss what they know of their latest case (a Town of the Week where everyone is suddenly immortal):

Sam: It seems like the last person to actually die around here was this boy a couple of months ago; we should probably start by contacting him.
(Beat)
Dean: ...I love how matter-of-factly you just said that. Our lives are weird.

The crew of Moya on Farscape are firmly in Type 2 territory. Crichton even lampshades it in one episode when he refers to aliens messing with their minds as "pulling a T'raltixx," in reference to a previous mindscrew. Scorpius is somewhere between a Type 1 and Type 3. Nothing fazes him. Ever.

Music

The song "I've Seen It All" from the musical film Dancer in the Dark, sung by the main character, who's going blind.

New Media

Play-By-Post Games

Shadowside: Few of the students of the Twin Campus would be surprised that the new guy is an anthropomorphic platypus or what have you.

Tabletop Games

Dwarf Longbeards in Warhammer Fantasy Battle Fantasy Battles are immune to Panic due to this.
What's New with Phil and Dixie has fun with gamers being somewhat jaded by weird stuff.

Video Games

In Knights of the Old Republic, Canderous Ordo, after The Reveal, makes a comment that, "Remember, we're talking about the Force here. At any moment, Malak could fall from the sky and I wouldn't bat an eyelash."
Happens a lot to anyone playing any MMORPG, due to the anonymous nature of the users. Anyone can say anything they want, and do, e.g. "I hope your (loved one) gets (expletive) and (action) down the (undesirable region)". Hence being called anything in the real world would perhaps earn an lol from any experienced MMO veteran.
In the DLC for Mass Effect 2, Shepard and Liara have become Weary Travelers.

Shepard: Be ready. I wouldn't be surprised if this summoned a Reaper.

Liara: How many guards does the Shadow Broker have?
Shepard: Told ya.

- In Mass Effect 3, Mordin's completely deadpan response to seeing Javik, who comes from a long extinct race, says it all:

Ah, a Prothean. Excellent.

Leon Kennedy in Resident Evil 4 has a Type 2 moment partway through the game. After one of the villains undergoes a dramatic One-Winged Angel transformation and emerges as an enormous monster, Leon scoffs "Monsters. At least after this, there'll be one less to worry about."
All four of the characters in Left 4 Dead are like this, which is shown in detail in the comic, where they constantly have to explain things to the (in theory better trained) CEDA officers. Left 4 Dead 2 shows the new characters gradually becoming this.

Web Comics

The former seneschal of Mechanicsburg in Girl Genius is type 1. "Don't try to boggle me, Mr Talking Cat. This is MECHANICSBURG. You are by no means the strangest thing in this town!"
- Dr. Sun in the last panel.
- Higgs as well. He's looked worried exactly twice: When he accidentally insulted Zeetha, and when two sparks are getting waaaay too excited about their "experiments."
- Paris is a big city. These people are worldly, sophisticated. They've seen everything. Well, almost everything.
Gunnerkrigg Court. Antimony was practically raised by The Guides, so she's almost completely unfazed by the supernatural. For example, compare Kat's reaction to Ketrak with Annie's complete lack of a reaction.
- Jones, also. Apparently the fact that she has only seen something similar is greatly disturbing. A chapter dedicated to her story later gives an idea exactly how disturbing this should be.
- While he didn't start off as this, there's Jack. His only response to a really pissed off Reynardine is "Oh... you have a, uh, large wolf with you. Okay. Cool. Nice flowers." He was visibly startled, but he took it in stride.
In The Order of the Stick, Xykon is a type 3. He does it while having difficulty remembering which guy named Fyron he killed in Cliffport.
Sarda of Eight Bit Theater is a type 2. He has literally seen everything that ever happened or ever will happen. He has learned every bit of magic ever. You can't beat him, though you can (very rarely) surprise him with extreme stupidity.
- By the end of the comic, Black Mage, Thief and Red Mage had become type 2s. They had even become aware of the basic jokes of the comic and could see them coming.
Sha'sana of Drowtales is one of the few surviving Dark Elves and combines several different types, showing very little expression, weariness at what the drow have become, and a certain smugness about the impending disaster of nearly every tainted drow in Chel losing control of her seeds while keeping Sharess' body in the Ninth Tower that made many fans dislike her.
The Inexplicable Adventures of Bob are inexplicable enough that Bob has reached this point. Jean is getting there.
El Goonish Shive shows that being an old friend of a fledgling Mad Scientist is enough:

Sarah: You don't consider goo coming to life news?!
Elliot: Not after some of the stuff I've seen at Tedd's house...

Ida, the Mayor's assistant from The Word Weary, doesn't bat an eyelash when her boss trashes his office and throws his phone out the window.
Schlock Mercenary had a moment when an employee of Urtheep Industries introduces another to the fascinating, but unsettling realities of Services and Return Division. And then, of course, things get weirder than ever before.
Sanity Circus has this little discussion:

Nimbus: Uh… It's a bit weird!
Attley: We're with a guy who's a flute, another with feathers for hair, and we're being chased by fear-eating monsters. "Weird" is a bit lost on us, now.

Web Original

Losing the ability to be surprised by entertainment is one of Cracked.com‍'‍s 5 Warning Signs That You're Finally Getting Older. Entertainment will ruin your life with or without tropes.
Anyone who reads this wiki (or The Other Tropes Wiki, for that matter) for too long runs the risk of becoming like this.

Western Animation

Jonah Hex is portrayed this way in Justice League Unlimited, where he not only isn't surprised by a group of time traveling superheroes from the future, but he instantly figures it out. And, to a lesser extent, a bit earlier in Batman: The Animated Series
Jonah's no less fazed in Batman: The Brave And The Bold, where being in the future on an alien world, being forced to track down various species to fight, doesn't warrant much surprise. Batman's no stranger either, and seems to take time-travel, galactic teleportation and mystic outer planes in his stride.
Kup from the Transformers movie and G1 TV series is a bit like this. He's always telling war stories of times that were like times like this (to Hot Rod's increasing irritation during the film)... until he sees Un ic ron.
The Grim Adventures of Billy & Mandy: Grimm seems to be a great example of this. There's the way he defeated Horror's Hand in one of the movies, and this quote:

Billy: This is scary! Let's watch something else! (turns off the TV)
Grimm: Aw, come on! I've seen scarier stuff in your toilet!

- Mandy is this in spades, and is even immune to irritation (as revealed in the episode with the invisible duck) due to being constantly exposed to Billy's pure stupidity-infused antics, which drove the goddess of chaos (Eris) insane.
Most of the characters is Venture Brothers are this, with Dr. Venture being a very tired type 2, Brock alternating between type 1 and type 2 depending on his mood, Hank bragging about his experiences putting him into type 3, and Dean growing from an enthusiastic type 3 into a very scared type 2.
- On the villain side of things, The Monarch is a type 2, Dr Girlfriend is a rare type 1, Phantom Limb is a type 3, #21 is a type 2 and #24 is a wannabe type 3. Honestly, Jonas Jr. sticks out because he's one of the few characters who isn't Genre Savvy enough to have Seen It All.
Skips in Regular Show says "(yeah,) I've seen this before" in response to pretty much anything.
South Park sees so many strange things that nobody bats an eye at anything short of the entire town being destroyed. A good example is in the episode "Mr. Garrison's Fancy New Vagina":

Officer: So let me get this straight. That woman over there was trying to get to her balls which were in the knees of a black child whose father is a dolphin.
Stan: Yeah, that's basically it.
Officer: Sounds like an open and shut case. All right, let's head 'em out!

Real Life

There is absolutely nothing you can confess to an experienced priest that they haven't heard before (with "experienced" meaning "has been in the Church for several decades").
- The guidelines for conduct of Mass say that, during a specific part of the Communion, stopping is not allowed. Thus, there is a manual required to be read prior to ordination. It includes everything from a fly landing on the host to gunmen taking hostages. It's mostly drawn from experience.
Some people who've worked in tech support have heard it all.
Master chess players are so good primarily because they have seen basically everything that could come up in a chess game.
Medical professionals as well. And they've heard all the weird explanations concocted by embarrassed patients hoping to avoid confessing to what actually happened, ranging from cutting oneself shaving to falling off of a ladder onto a lamp (because he was dusting a ceiling fan while not wearing pants).