Kilroy Was Here

So you're somewhere—on vacation, at war, time traveling, teleporter accident, whatever—and you want something that'll stick around longer than just a Fun T-Shirt. That's easy, just vandalize a handy surface!

This trope is pretty much as old as humanity itself (it's posited that one of the reasons there are hand-prints at Lascaux is for this very reason), but the Trope Namer is a bit of Memetic Mutation from World War Two whose origin has many stories (see Real Life section below).

Bathroom Stall Graffiti and Sweetie Graffiti could be, in a sense, be considered subtropes.

Do not confuse with the Styx album.

Examples of Kilroy Was Here include:

Anime and Manga

Pokémon: The memetically infamous "Gary was here! Ash is a loser!"

Fan Works

In chapter 11 of New Perspective Evangelion Kaworu discovers a Kilroy graffito in the entry plug of Asuka's Evangelion.
In Drunkard's Walk II, a freed Boomer takes "Kilroy" as his name and uses the graffito as a marker to indicate where other Boomers can find deposits of the nanite/organism which frees them from GENOM control.
The story Back in my day! from The Teraverse hints that Kilroy was actually a 1930s-40s vintage Mystery Man, "Kilroy the Barbarian", who "had been practically everywhere".

Literature

Journey to the West: Sun Wukong makes a bet with the Buddha that he can leap out of Buddah's grasp. He goes an extreme distance away and finds a set of pillars. Thinking that he'd won, he leaves the words "Son Wukong Was Here" on the middle pillar and urinates on it for good measure. Turns out the "pillars" were Buddha's fingers. Oops!
In The Neverending Story, there is a mountain that is only conquered for the first time - that is, you are always "the first person to ever climb it"; it is impossible to get on the top as long as there is any memory of anyone doing it before. So leaving a tag at the top blocks everyone else from it until the tag is worn out by the elements.
In The Shawshank Redemption, inmate Brooks, unable to fit into society after so many years in prison, carves "Brooks was here" into a ceiling beam from which he then hangs himself. At the end of the film, Red carves "So was Red" before leaving to join Andy in Mexico.
Isaac Asimov's short story The Message is about a time traveler from the 30th century to WWII North Africa who, forbidden to interfere (he can only observe) resorts to writing a message on the wall of the hut he was in. His name is George Kilroy.
The Sweetie Graffiti left carved into a boulder for Peter by Charity in William M. Lee's short story "A Message From Charity" as well as its 1985 Live Action Adaptation on the revived The Twilight Zone series.

Live-Action TV

In My Name Is Earl, Randy had made several graffiti like this, and after the episode where he goes back to school, he sets Earl to correct them since he [Randy] has realised that those graffiti had bad spelling.
Two Kilroy faces appear in the opening credits of Community, with the noses forming the l's in "Joel McHale".

Newspaper Comics

In one Footrot Flats strip, Wal makes a moving speech about how insignificant it makes one feel knowing that an ancient tree will still be standing centuries after he's dead. He then carves "Wal Footrot Was Here" into the trunk.

Radio

In the radio program The Black Museum (based on real cases of Scotland Yard and real objects in their 'Black Museum'), episode 'The Notes', police find notes actually saying 'Kilroy Was Here." The police eventually realize the killer is actually telling them his name is Kilroy!

Video Games

In general, any video game that lets you leave behind some kind of message (text or graphical) can result in this trope.
- This, and bragging rights, is the whole point of high score lists.
Left 4 Dead has graffiti in every safe house as a form of pseudo-communication between groups of survivors. Among them is Chicago Ted, whose statistics for zombie killing become something of a legend.
In Pokémon Ruby and Sapphire, outside Lilycove City's Pokémon Fan Club building, it is noted that a number of Trainers have scribbled their names all over its sign. There is also a small island in the waters by Seaside Cycling Road that Team Aqua (or Magma) have tagged ("Team Aqua Was Here!").
Subnautica lets you leave behind a gift as well as a message for a future player of another instance of the game.

Web Original

Homestar Runner: "Rad, rad Robotank!"

Western Animation

On the Looney Tunes short "Haredevil Hare", Bugs Bunny lands on the moon and mentions that he's the first living creature to do so. He then passes a rock with "Kilroy was here" written on it.
The Simpsons: Bart has made several "El Barto was here" graffiti.
- In another episode, Bart writes his name in wet concrete, with an Imagine Spot of the future, where archeologists speculate on what he might be like.
- Homer tries doing this using the name El Homo, which results in him getting hit on by a man passing by.
In The Boondocks, Riley graffito-tags his name in big bold letters, then wonders how he got caught.
In The Flintstones episode "Adobe Dick", Fred and Barney are swallowed by a whale. Inside, they spot a graffito that reads "Jonah was here".
At the end of The Fairly OddParents TV Movie "Channel Chasers", Vicky's sister Tootie gives Timmy's Parents pictures of all the damage Vicky has caused over her time as babysitter. One such picture displays just this.

Timmy's Dad: Vicky was key-scratching "Vicky Was Here" on my car! I punished Timmy for key-scratching "Vicky Was Here" on my car!

Real Life

As mentioned above, "Kilroy was here". However it originated (The Other Wiki has a few theories), American soldiers began leaving Kilroy everywhere.
Graffiti tags. There's even a "language" associated with specific artists and gangs depending on what elements are present in a tag.
Places like Names Hill, Register Cliff, and Independence Rock in Wyoming; according to Wikipedia, people on the Oregon Trail and the like carved or had carved their names in them. Names Hill also has Native American pictographs.