Everybody Did It

A famous detective has gathered a group of likely suspects. They analyze the evidence, interview the suspects, and thoroughly investigate the crime scene in order to figure out which of the gathered people is the culprit. The conclusion? Everybody was.

This is Everybody Did It. As opposed to having one or two people commit a crime out of several potential suspects, all the suspects were in on it to some extent.

Not related to the other kind of "doing it." Usually.

As an Ending Trope, Spoilers ahead may be unmarked. Beware.

Examples of Everybody Did It include:

Anime and Manga

A filler episode of the Ranma ½ anime had an investigation of "who stole the takoyaki?" complete with Shout Out to a famous samurai mystery drama series (famous in Japan, that is...)
Chapter 30 of Franken Fran is set up as a typical murder mystery, and sure enough, people start getting attacked, although Fran manages to keep them alive. It turns out that they're injuring themselves, because they enjoy having Fran operate on them.
Seems to be part of the point that the various narrators are trying to make in Umineko no Naku Koro ni. Any time the parents find the gold, they will inevitably begin offing one another like crazy in a bid to keep it for themselves, making every single one of them possible culprits.

Comic Books

Peter Milligan's Shade the Changing Man used this to avoid solving the mystery of "Who Shot JFK?", instead Hand Waving with a glancing look at every possible speculation, then concluding that Everybody Did It. Justified in that Shade is a stranger to American culture, and that he was dealing with a madman's obsession covering up for grieving his lost daughter.

Film

The murders committed in Hot Fuzz were pulled off by the NWA (Neighbourhood Watch Alliance), which consisted of almost every named character in town.
Played for Laughs in the climax of The Pink Panther movie A Shot in The Dark. Clouseau's interrogation goes completely out of control as the different suspects start bickering amongst each other and shouting accusations; from this, he is somehow able to deduce that "they were all murderers, except for Maurice, who was a blackmailer!"
This is how Blair Witch 2: Book of Shadows ends... maybe...
Inverted- In the 1955 Hitchcock film The Trouble with Harry, a town of people find the body of a dead resident, Harry. Everyone believes themselves and everyone else to be responsible for the death. It turns out that no one did it, and Harry died of natural causes.
The third ending of Clue. All but one of the surviving characters end up having killed someone, as does one of the victims. And it even makes sense in context.
Half the town tried to kill the eponymous character in Drowning Mona, though most of them were pretty incompetent about it.
Gosford Park: the Asshole Victim was the father of the would-be murderer by his housekeeper, who knew her son was planning to kill him and got there first so that he'd be guilty only of stabbing a dead body.
The film version of Murder on the Orient Express is an even better example of this than the book.

Literature

The former Trope Namer would be the Agatha Christie novel Murder on the Orient Express, where everyone was suspected, and for good reason. Not really a spoiler any more. Also the Trope Maker and Ur Example.
Subverted in the Randall Garrett Lord Darcy story, The Napoli Express (whose name is an obvious shout-out to the Christie novel). When the non-hero detective comes up with the "they all did it" theory, the hero has to restrain himself from saying how silly the idea is. The people involved can't even hide that they all know each other, even though they're trying to. Hiding that they conspired together to commit the murder is quite ridiculous.
- The whole story is a Shout-Out to the Christie original. Garrett's solution even harks back to the one Poirot presents as the alternative to the trope.
Reginald Hill's Dalziel and Pascoe novel parodies this trope in Pictures of Perfection. Virtually every major character commits some sort of crime, except for murder. Much to Dalziel's irritation, nobody wants to file charges against anybody else.
Older Than Print: The Trouble with Harry (above) is based on "The Tale of the Hunchback" from The Arabian Nights.

Live-Action TV

Comically subverted in the British spoof anthology series Murder Most Horrid starring Dawn French: in the episode "The Case of the Missing", everyone did do it, but the detective assigned to the case (French) is so confused and frustrated by their manipulation of the evidence that she finally snaps and concludes that she must have done it.
- Subverted again in the episode "Mangez Merveillac". Obnoxious travel writer Verity Hodge (French again) makes the French town of Merveillac a hugely popular tourist destination by forcing the locals to conform to stereotypes. Eventually, the locals grow tired of this, murder Hodge, and serve her up to the tourists at a local festival. However, once the credits roll, we discover that Hodge is actually alive and well, and the "murder" was part of a scheme to draw even more tourists to Merveillac by inspiring a Hollywood blockbuster.
In The L Word, at the end, Cathy could have been killed by anyone.
In Star Trek: Deep Space Nine, this is basically how Cardassian mystery novels are said to play out, with all the suspects being guilty; the mystery is figuring out who is guilty of what.
The episode of Monk where he went to a vineyard. He meets a guy, but the next day, everybody denies that he existed. The guy stole money from his employers and died of a heart attack. The other guests find out and agree to split the money, but hid the truth from Monk because he's honest, so they erase all traces of the man's existence.
- Another case involve a bank robbery in which the entire robbery was staged by all of the staff in bank.
This was used in an episode of Cold Case—specifically, the one about the virginity club. The victim knew something incriminating about each other member of the club, so they decided to kill her together. By stoning, no less. (Yes, it was a touch Anvilicious.)
One tongue-in-cheek episode of CSI concerned a man found dead in a swimming pool. Every time Doc Robbins finds a possible cause of death, it looks like a new suspect is to blame for his injuries, whether for intentional homicide or reckless endangerment. Eventually it turns out that the repeatedly-battered man sat down to recouperate, slipped into the pool when his deck chair collapsed, and accidentally drowned, meaning Everybody Failed To Do It.
- And played straight in "Unfriendly Skies", when mob rule took over and they all had a hand in killing the guy.
The Jonathan Creek episode "Satan's Chimney", wherein the second murder victim was killed for inciting the murder of the first victim, by the first victim's friends.
Played for Laughs in the Ripping Yarns episode "Murder at Moorstones Manor," which ends with a standoff between the characters claiming credit for the murder.
In an episode of Foyle's War, the victim is hit over the head with a rock and then drowned in a trough. It turns out these assaults were committed by two different people, and witnessed by a third would-be assailant who never got his chance to do anything. The rock-wielder is let off, with Lampshading to the effect that he's just lucky half the town was out to get the guy that night.

Theater

The JB Priestley play An Inspector Calls features this trope to a degree. Although the girl committed suicide, the entire Birling family drove her to it one way or another, and this drives the acceptance of social guilt that Priestley wrote the play to emphasize.

Video Games

In Ace Attorney Investigations, Callisto Yew claims to be the Yatagarasu before escaping custody. However, Kay claims the Yatagarasu was her father. Turns out neither is quite true. They were both members of the Yatagarasu--as was Detective Badd.
Part of a Jedi test in Knights of the Old Republic. Two people are suspected of killing a man, when it turns out that both of them intended to do so independently of each other, but one of them (non-fatally) shot the other conspirator by mistake when he thought he was the victim, which allows you to find the truth yourself.
In the Hidden Object game Madame Fate, all of the suspects are revealed to be plotting against the fortune-teller and/or one another. Subverted; someone else kills Fate -- and all the suspects -- before they can enact their schemes.

Western Animation

Played for Laughs in the All Just a Dream Daria episode "Murder, She Snored", in which Kevin is murdered and Daria is the main suspect. As it turned out, everybody but her did it, all unaware of each other's plots and thinking they were the sole murderers. Even though they all fess up when Daria starts pointing fingers, Daria gets convicted anyway.
In What's New, Scooby-Doo? the guest stars are chased by an invisible madman and slowly proven that each suspect has an alibi Velma concludes that the Scooby-Doo Hoax was a group effort by the suspects, using a technology they were developing for the government.
- Also in "Mystery of the Samurai Sword", the Scooby-Doo Rule that "the first fully named character did it" is both played straight and subverted - ALL the named characters are in on the plot.
Done in an episode of The PJs, Thurgood takes Calvin and Juicy's homemade go-cart for a ride and wreaks it in the process. He plays innocent while the residents try to get to the bottom of who broke it. To which each one admits they had taken it for a joyride in some form or another.

Real Life

Execution methods such as firing squads and stonings rely on the idea that if everybody kills someone (or at least, a group of people do it) then no-one needs to feel guilty because no-one knows who finished them off.
When Julius Caesar was assassinated, each of the dagger-wielding conspirators made a point to strike a blow.
Ancient Rome had the Year of the Four Emperors (69AD), when the top spot was occupied by four different Emperors in succession (in order, Galba, Otho, Vitellius, Vespasian). To quote the Other Wiki:

Altogether, around 120 people claimed the credit for killing Galba, being anxious to win Otho's favour and hoping to be rewarded. A list of their names was drawn up, which fell into the hands of Vitellius when he succeeded Otho as emperor. Every one of them was executed.

When the YMCA or similar organizations hosts a murder mystery game for kids, they'll use this solution. What's more, there will be evidence pointing to everyone. Presumably, this is so no matter what you guess (unless you guessed suicide), you're never completely wrong, and all the kids go home feeling more or less satisfied.
Adolf Hitler: He was so batty that he managed the seemingly impossible feat of getting the British Empire, the United States, and the Soviet Union, as well as many lesser powers all to agree that he would look better on a gallows then as head of a major nation. When someone gets to the point where you can almost say that everyone in the world did it then you can be pretty sure he is unique.
- Officially he committed suicide. So technically even Hitler did it.