Pulp Fiction/WMG

The Golden Fleece is in the briefcase.
See The Spirit. Or The Spirit.

A collection of Oscar statues is in the briefcase.
They're from an alternate universe where Pulp Fiction swept the Oscars.

Marsellus Wallace's soul is in the briefcase.
Classic theory. The briefcase contains the soul of Marsellus Wallace. The devil tore it out of the back of his head (hence the Band-Aid). Note the password on the briefcase is 6-6-6. It glows brightly and is beautiful enough to utterly disarm nearly anyone who looks into it.
 * Something to point out that when Jules and Vincent kill Brett, the same color of the glow from the briefcase flashes across the screen. Soul leaving the body?
 * Marsellus Wallace sold his soul for the power to run his criminal empire. But then he meets and married Mia... and the only way he can be truly fulfilled is to have his soul back, which he will pay any price to do - hiring the Wolf can't be cheap. Funny thing about souls, they are what lets people truly feel. Marsellus Wallace is a hard, ruthless man, having sold his soul, but part of the price of regaining it is to feel all the emotions that were numbed by its loss. Marsellus, the hard, ruthless criminal kingpin, must have caused immense anguish and suffering which would be reflected in his soul, if he had one. Then he regains his soul, and the suffering begins. (At the same time he's having his soul reseated, Mia Wallace overdoses on heroin and dies - whoever Marsellus sold his soul to wants it back, and takes Mia's soul in exchange.) A soul sold to the devil is cursed by God, and brings ill fortune to whoever possesses it. Only Jules Winnfield walks away whole, a man freshly set on a path of righteousness - judged, and declared worthy of redemption.

The jewels from Reservoir Dogs are in the briefcase, and Mr. Pink is the waiter at Jack Rabbit Slims.
He survives the shootout at the end and has apparently escaped custody and is, now incognito, trying to get the jewels back.
 * Also, Pink's real name is Carl Showalter.
 * If the jewels are in there, why does it glow?
 * Special light for a special case. It could have been lined with gold foil and a light bulb that turns on when opened along with the diamonds.

Mr. Pink and Mr. Orange are both alive and in this movie. And the briefcase contains the diamonds from Reservoir Dogs.
Mr. Orange survived being shot. After all he survived that long already and we never see where he's been shot. Plus people have survived shots like that. Mr. Pink co-operates with the police, tells the location of the diamonds and gets parole. After all we never see him kill anyone and he's the only one without a back-story. For all we know he could be scot free other than the robbery. So then Mr. Orange gets a new identity, that of a small time crook to retrieve the diamonds and hooks up with Amanda Plummer. He gives back the case at the end because he knows Jules and Vincent both work for Marsellus.
 * You think Orange survived being shot in the head?
 * Mr Pink killed a few policemen at the start of the film, so I doubt he would be let out so easily.
 * Some theories, due to ambiguity since the last bits of Reservoir Dogs only show Mr. White thinking about shooting Orange, say that White was killed before he could plug Orange. Therefore, it could be plausible that Orange survived. Not to mention The Bride survived a freaking gunshot wound to the head.

The briefcase contains the cocaine Clarence stole in True Romance
Not all of it, naturally -- alot of it was destroyed during the shootout at the end of that movie.

The briefcase holds the same top secret mystery as the trunk of the car in Repo Man.
Both give off a golden glow, etc.
 * Impossible. The glowing thing in the Trunk in Repo Man kills who ever looks at it.
 * The people who do look inside it all die. If I'm not incorrect that is.
 * You are incorrect. The guy in the diner looks into it and survives.
 * For now.

The briefcase contains the remains of the Ark of the Covenant, or holds the contents of the Ark, and is itself a form of the Ark, as described in "Raiders."
The Ark makes any army marching with it invulnerable, which explains why Jules and Vince are able to have weapons fire at them at point blank range with no injury (while holding the briefcase). Also, the briefcase shines with an interior light, like a weaker/smaller/more aged version of the opened Ark in "Raiders."
 * Vince survived looking into the briefcase. This might mean the briefcase is the Ark from the Xenaverse. While the Indiana Jones Ark kills whoever looks at it, the Xena Ark kills whoever looks away from it.
 * But Vince did die by the end of the movie. The Ark may have "cursed" him to be shot by Butch rather than melting his face off.

The briefcase contains a lightbulb.
Oh come on! Can we please keep it somewhat plausible??
 * Seconded! The glow might be just a light built into the suitcase, like the ones that you see in glove compartments sometimes. When you accept this theory, the possibilities of what people are really reacting to in the case open up quite a bit.
 * Maybe it's the Most Valuable Lightbulb EVER MADE?
 * Self illuminating light bulb, therefore it would be a self-powered one which would be a stone's throw away from perpetual energy/free eternal lasting energy. Which leads me to believe Marsellus Wallace and his gang work for Greenpeace.
 * Word of God: It's a lightbulb. A supernatural one. At least, it is if you accept that Watson's first and middle names were Arthur Conan.
 * No, Word of God said that in real life they used a yellow-orange light bulb as the prop. However, there has never been any Word of God explanation for what is in there in-universe and presumably there never will be. See the entry on The Walrus Was Paul.

The briefcase contains a gigantic onion.
It gives off a golden glow, brings tears to some characters' eyes...

Let's be realistic here: the briefcase contains gold bars.
What else would mean something to a mobster, would be carried in a discreet briefcase, and glows golden when the briefcase is opened? Characters getting emotional about its content is simply their greed overcoming them.
 * Gold Bars are heavy; it's difficult for even one to be carried, let alone a couple in a suitcase. The object(s) in the suitcase must be lighter than Gold Bars, yet similarly or even more valuable.
 * Or, they could be gold leaves.

The briefcase contains Batman.
Glowing Batman, who is about to give them all the beating of their lives; seeing him in there just fills everyone with awe. But he's only two inches tall, and it's the middle of the day so he's not so scary, and so they can just close it up again. Tiny little glowing awe-inspiring Batman!
 * Tiny little glowing awe-inspiring Batman? You, sir, win.

The briefcase contains a small, active nuclear bomb
Yes, it will kill those who look upon it - but it'll take a while...
 * Already been done.
 * But of course. Where do you think Quentin got the idea in the first place?

The briefcase contains more than a film canister's worth of marijuana.
That '70s Show tells us that a film canister full of marijuana glows bright green. A suitcase full probably fills the mana gauge completely and starts glowing gold.
 * Alternatively, it is a Gold Herb from the Resident Evil series and the marijuana in That 70s Show was just a lesser Green Herb. Although, that suggests the herbs in Resident Evil are marijuana, which would explain a lot...

The briefcase contains something with only sentimental value, and is an exercise in "trust" and the nature of human perception.
This might seem a little odd, but the briefcase contains something rather mundane that belongs to Marsellus - perhaps some cheap jewellery, or some kind of souvenir. Vincent and Jules become transfixed upon looking at it, not because it's something particularly special in itself, but because they're trying to work out why Marsellus finds it important: they reason that Marsellus is an important and dangerous man, who must therefore want this object only if it is important or dangerous. Similarly, "Pumpkin" observing "It's beautiful" might simply be trying not to offend the guy pointing a gun at him.
 * The Gold Watch sequence gives credence to this idea.
 * But Pumpkin calls it beautiful before he knows Jules has a gun on him.
 * Not the OP, but if we accept this theory, Pumpkin could simply be a man who has a genuine appreciation for the smaller things in life. He's clearly a man with at least a few hidden layers beyond what we see; for one, he's an Englishman in America, so for all that he's an impulsive criminal now he probably had some motivation beyond small-scale robberies for moving out there, which hints at possible backstories and ambitions. Furthermore, despite his criminal tendencies it's clear he deeply and genuinely loves Honey-Bunny. Like Marsellus, who finds something of value in this seemingly small object that the ruthless, cynical criminals he hires can't see, Pumpkin is also capable of seeing something of value and beauty in it. By this logic and reasoning, it adds to a note of hope and redemption to Pumpkin and Honey-Bunny's story, that like Jules they have the possibility of walking away from their grungy and seedy criminal lives and rising above it to make a more meaningful life for themselves.

The briefcase contains Butch's gold watch.
Marsellus became aware of Butch's attachment to his father's watch, and so he hired someone to steal it. The guys whom Jules and Vincent were hired to kill tried to pull a fast one on Marsellus and grabbed as much stuff of his as they could find - including the case, which Jules and Vincent later returned. Marsellus expected that Butch would eventually mention to him that the watch was missing; he would then promise to find it and shortly thereafter produce the watch. Marsellus presumed that Butch would feel indebted to him and less likely to screw him over. Apparently, his plan failed.
 * If Butch's gold watch was stolen, then why was it right where he left it in the apartment?

The necklace from Titanic is in the briefcase.
Impossible. In the music video for "Oops, I Did It Again", Britney Spears as the queen of Mars clearly collected the necklace so she could give it to her astronaut boyfriend.


 * To be fair, the movie is set in the mid-eighties to early nineties, before Britney Spears was more than just a wee Mousekateer and before Mars landers were pretty common. It's possible that the necklace eventually made it's way to Spears by this time, after getting tossed around in a massive mafia war that killed or incapacitated Marsellus and got into the hands of a music exec who wanted to reward the new pretty to the first pop mogul to set up a fanbase in space: an alternate universe Britney Spears, who instead of going mad and having a massive meltdown in 2007, decided to go into astronaut training.

Samuel L. Jackson's wallet is in the briefcase.
The one that says Bad Motherfucker.
 * So where did the wallet in the bag come from?

The briefcase contains another briefcase.
It's just briefcases all the way down.
 * "What's in the smallest breifcase?" "Porn."
 * Is one of them a Clash of the Titans lunchbox?

The briefcase contains The One Ring.
Everybody wants it, and it transfixes those of weaker wills. Some people are willing to kill to have it. Jules is made of stronger Motherfuckin' stuff, and so has no desire for it and will give it to his boss willingly. Oh, and the Ring isn't likely to let its new bearers be killed by something so mundane as a fellow with a BFG.
 * This, of course, means Jules is really Tom Bombadil.
 * Ah, that would explain a lot. Oh, no, wait, that just raises further questions.
 * Okay, Tom Bombadil being played by Samuel L Jackson is now officially the most awesome thing ever.

The briefcase contains a permanent, object-bound Hypnotic Pattern spell.
Its only value is that it glows and looks hypnotically beautiful to anyone who sees it. The briefcase reduces the effect it has on the holder, so that it can be aimed and used by a non-spellcaster, or simply looked at and enjoyed without becoming unable to ever close the suitcase.

The briefcase contains whatever Fry always wanted
Tangled up Christmas lights, and wide-mouth booze.

The briefcase contains the G.E.C.K
The film takes place in the Fallout universe - or maybe Fallout takes place in Pulp Fiction universe, especially since both pay homages to popculture. That's why you don't see computers (because they would be too huge and limited to bases and factories) and everything has that old-style feeling. It takes place shortly before 2077. Butch ran away from Marsellus to find a suitable vault. The game also provides a Shout Out to it by having a boxing ring in the second game. Marsellus needed the G.E.C.K to create New Reno when he left the vault. And the motive for Bishop's revenge on Vault City was because Lynette stole Marsellus' G.E.C.K to create the city and left him with only fundaments. Also, look at Angela Bishop's sprite - black hair, addiction to Jet, is related to a mob boss... she's an Expy of Mia Wallace! Jules after the incident with the "miracle" realized that he isn't doing a plain ol' job. He now knows that what he protects is for ensuring a better future.
 * Lynette is younger than Vault City. Of course, it might be an inherited feud, with the Bishops keeping a grudge against Vault City and its leadership.

The briefcase contains the Eye of Harmony, which Marsellus Wallace stole from the Time Lords.
That golden glow looks suspiciously similar to the one witnessed by Rose Tyler in "The Parting of the Ways". Just saying...

The briefcase shows each person his own destiny.
Everybody who looks into it is transfixed, but has a different reaction. Vincent looks confused and almost scared because it shows how he dies. Honeybunny and Pumpkin see that they give up their lives of crime and live peacefully thanks to Jules, hence "It's beautiful." Jules never actually sees what's inside, so he's able to become a free man with no knowledge of his future.

The briefcase contains Gold Kryptonite
Which is why it's so highly vauled in the criminal community.

The briefcase contains a Piece of Eden
And Jules and Vince are Templar agents attempting to get the Briefcase from Marsellus to deliver to Abstergo Industries.
 * And after repenting, Jules joins the Assassins. Thus, we have Assassin Samuel L Jackson.

The briefcase contains a lot of drugs. The glowing is a metaphor.
Really now, we're talking about a crime lord and his hitmen, some punks with the guts to steal from a crime lord with hitmen, and some small-time muggers. A suitcase full enough with drugs is definitely enough to hold any of their attentions.

The briefcase contains Wallace's dirty laundry.
Jules wasn't lying!
 * Specifically, that shiny, really high-quality commissioned over-the-shoulder thing he was going to wear at the end of the week while out with Mia Wallace. Jules wasn't being facetious, he was asking if Wallace was in drag when the guy saw him or not (apparently not, from the confusion).

The briefcase contains Wallace's dirty laundry... metaphorically.
It's incriminating evidence that needs to be taken to an associate without falling into the wrong hands, but is useless to anyone who isn't the cops (since they'd be killed if they tried to tell the police, and would probably not be believed anyway). The people who understand it are amazed at the information, the people who don't are puzzled, and Pumpkin thought that the embossed gold-leaf seal was beautiful. The glow is the lightbulb in the case reflecting off of the contents, including the aforementioned golden seal.

The briefcase contained the Source, the Eden device from series 4/30.
Warning: Direct exposure to the Source may result in unwanted regeneration immediately following such injuries as gunshots. If you are exposed to the Source, please contact your local terraformist medic for immediate treatment. The Source is not to be handled by fish-men, gangsters, or sad onions under the age of three.

The briefcase contained a Golden Ticket.
I can't take credit for this one: an old cyber pal of mine on a now long since defunct Tarantino forum came up with it. But I always liked the notion. Don't know how much a Golden Ticket would glow but I'm sure there are ways to Hand Wave your way out of that (I wouldn't care to try) and can't you just see Quentin Just-Call-Me-Mr.-Humorous-Homage doing it?

The briefcase contains stacks of photographs, and they're all pictures of feet.
Look, this is Tarantino we're talking about.

The briefcase contains either black mask issue #1,the maltese falcon or the mask that gave its name to the magazine..
Since the working title for pulp fiction was black mask.

The Briefcase is God's personal minifridge.
This explains the golden glow when it's opened, as well as the "miracles" bestowed upon those who carry it. People shed tears of joy upon looking at its contents: an unlimited amount of root beer cans, the drink of the gods.

The briefcase contains plutonium.
Doc Brown told the Libyans to steal the plutonium and drop it off at a safe house. The plan was once the authorities stopped looking for it, he'd pick it up to make a nuke. Then, he paid Marsellus to send his men to the apartment and make it look like the mob stole the briefcase full of plutonium so they wouldn't come after him for not building a nuke. Somehow, they figured out he paid Marsellus and as a last ditch effort, offered them a nuke shell full of pinball parts to buy more time for building the Delorean.

The briefcase contains Tinkerbell.
It shines brightly. It's described as beautiful. It's very lightweight (look at little effort Vincent uses turning it over when he puts it on the counter in preppy kids' apartment). Obviously the preppy kids are Lost Boys who owe alot of money to Marsellus, but they don't have the money so they catch Tink, tie her up and put her in the briefcase, then they kick back, eat some Big Kahuna burgers and wait for the pick-up. KNOCK KNOCK.

The briefcase is Pandora's Box
The light is Elpis, the hope that remained in the box after all the world's evils escaped.

The briefcase contains a recently stolen and very valuable artifact from a Museum, Palace or Temple
The theft was very high-profile: "Is that what I think it is?"

== The briefcase contains a certificate of authenticity as a prop from Dial M For Murder or one of several Alfred Hitchcock Presents... episodes. == It's not what Ringo/Pumpkin thinks it is- Either a forgery of the actual briefcase, or not the briefcase from {=D Mf M=}. Whichever it is, he's a Know Nothing Know It All.

The briefcase contains Marsellus Wallace's birth certificate.
Pumpkin thinks it's a birth certificate. Honey Bunny just thinks it's some sort of shiny, awesome certificate of coolness because of the gold edging/frame that was added after-the-fact.

The briefcase contains the Cosmic Cube.
And we finally see it in the Stinger of Thor.
 * It all makes sense now. Jules quits a life of crime and takes off with the Cosmic Cube and retires as a piano player, only to be shot during the wedding massacre... right in the eye, in fact. He then joins the US government, using the Cosmic Cube as leverage to become the leader of SHIELD.

The briefcase contains... nothing.

 * It's just a really nice briefcase.

What color is a Pumpkin?

 * Mr Orange is now Honey Bunny's Pumpkin
 * To add on, the cops at the end of Reservoir Dogs shot Mr White, realizing he'd kill their mole.

The Katana in the pawn shop belonged to Bill.

 * Or to The Bride, who pawned it after giving up her life as an assassin at the end of Pt. II.
 * Or Budd, since he admitted to pawning his Hanso sword sometime in the past.
 * Technically Budd never said  However,
 * My guess is that its Vernita Green's (Copperhead), who was A) living in Pasadena, and B) didn't have her sword, either, when the Bride came calling.
 * My guess is that its Vernita Green's (Copperhead), who was A) living in Pasadena, and B) didn't have her sword, either, when the Bride came calling.

Marsellus Wallace and his gang work for Greenpeace.
And the group of people Jules and Vince needed shotguns for at the start of the movie were PETA or another similar organization, while Butch represents greenhouse emissions (saving Greenpeace by giving them a necessary evil)

The briefcase passes judgment on all who touch it.
Of the seven people known to have touched the briefcase, five of them are now dead, and one was brutally raped. Several of the deaths happened under odd and improbable circumstances as well; one was armed with a big gun, got the drop on Jules and Vincent, and missed every shot he fired at point-blank range at them, Marvin died when a gun casually waved at him was accidentally fired, and Vincent only died because he happened to be in the bathroom at the wrong time and Butch wanted some Pop-Tarts at the wrong time, not to mention the randomness of Marsellus' ending up in Maynard's shop. The only one who escapes unscathed is Jules, who had decided to turn away from his life of crime; hence, the suitcase spared him.
 * So you're saying that the briefcase is the Ark of the Covenant? And Jules survives because he quotes scripture?
 * And repented of his sins, went out and sinned no more.
 * What about "Pumpkin" the robber?
 * Pumpkin technically never actually touched the briefcase, but the briefcase passed its judgement of him via Jules -- as a fairly low-level criminal who, presumably, never hurt anyone (we don't know what the sins of the kids who Jules and Vincent liberated it from were), he was found worthy of surviving for another day. Presumably after this little scare, he and Honey Bunny seriously reconsidered their lives of crime as well.

Jules is an angel.
Divine Intervention stopped the bullets, because Jules himself stopped them. He doesn't like when people blaspheme, he quotes the Bible, and he attempts to save all he can. Marvin and Vince were failed attempts, while Jimmie, Pumpkin, and Honey Bunny were successes. Brett, Flock of Seagulls, and Hand Cannon were agents of the Devil, Marsellus Wallace's soul is in the briefcase, and Jules was sent down to restore order and save all he could.

Butch is the one who keyed Vincent's Malibu.

 * Confirmed in the commentary, actually.

Vince's death was a Batman Gambit by Mia Wallace.
As Mia said, if Marsellus found out about what had happened while he was away, they both would have been in big trouble. Mia had only known Vince for one night, and maybe he seemed trustworthy, but Mia couldn't know that. So two days later, when Marsellus plans to go to Butch's apartment to ambush him, Mia makes a remark to how good a guy Vince is, and convinces Marsellus to take Vince along. Then there's also the fact that, during the scene at Jackrabbit Slim's where Vince questions her about the incident with Tony Rocky Horror, Mia seems to know a lot more than she lets on.
 * Alternatively, she intended for Marsellus to die at Butch's hands, because if Marsellus were to die, Mia would be in line to inherit control of Marsellus' mafia empire.

Vincent contracted an STD while in Europe
However, he is unwilling to tell anyone the truth. He uses his own needles so no one else will get it from him. He resists the temptation to sleep with Mia Wallace, not because he'd get in trouble with Marsellus for that alone, but because he really doesn't want Mia to get the STD as well, which would get him immediately killed by Marsellus, no questions asked. When Vincent is at Jimmy's and the Wolf tells him and Jules to strip down to their bare ass, he hesitated, not sure if anyone else would recognize the symptoms. Later on, Vincent did nothing to stop Butch from gunning him down, knowing that it was either that or die slowly and painfully to the disease. "Mia: You can use my straw, I don't have cooties.
 * This also explains why he didn't want to use the same straw as Mia.

Vincent: Maybe I do."


 * It could be syphilis, and disease-induced dementia (rather than cheap, sometimes legal Dutch drugs) is the cause of his stupidity and disturbing lack of professionalism during his first post-"vacation" job.
 * Syphilis takes a long time to get to that point, though. It's possible, but he'd have to have contracted it before his time in Europe.

Marsellus Wallace took part in the sleep eradication experiments from the X-Files episode "Sleepless".
That's why he has the elastoplast covering the scar where part of his brain stem was removed. The briefcase contains the drug subjects had to take to boost their serotonin levels.

Fabienne was actually paid by Marsellus to set up Butch.
How else can you explain why she "forgot" Butch's watch and why Vincent happened to be there when Butch went to retrieve it?

The movie takes place during a Groundhog Day Loop
There are a couple minor variations in the two scenes that are shown twice-- In the diner, "Honey Bunny"'s line is slightly different the second time- from "Any of you fucking pricks move, and I'll execute every motherfucking last one of you!", to "Any of you fucking pricks move, and I'll execute every one of you motherfuckers!". Also, in the preps' hideout, The first time Jules and Vince kill Brett, there's no bullet holes in the wall- The second time through, they're there even before the "divine intervention" scene.

Marsellus Wallace is the twin brother of Dutch.
Dutch and Marsellus are identical, in both looks and personality. Both are quiet, soft-spoken, slow-to-anger, bald big scary black men who lead a moderately-sized organization. Black Lagoon and Pulp Fiction take place only a few years apart, therefore making sense that they'd still look the same.

Jules is a Beatles fan.
There's a deleted scene where Mia Wallace presents her theory that there are two type of people in the world -- those who are fans of Elvis Presley first, and those who are fans of The Beatles first, and while you can like both, you always love one more. In this scene, Vincent identifies himself as an 'Elvis' man. In many ways, Jules is presented as the exact opposite to Vincent; Jules is black, Vincent's white, Jules is apparently clean (excepting his enthusiasm to go to Amsterdam to try the readily available pot) while Vincent is a drug-addict, Jules is spiritually-inclined where Vincent is a skeptic, Jules is more respectful and thoughtful where Vincent is more abrasive and impulsive, Jules is a bit more competent where Vincent is a bit more clumsy, and so on. Given this theme of opposites within their relationship, it would make sense that where Vincent identifies himself as an 'Elvis' man, Jules would be more inclined to identify himself as a 'Beatles' man. Furthermore, upon hearing Pumpkin's clear English accent, Jules constantly addresses him as 'Ringo', referencing the Beatle of the same name -- now, you obviously don't need to be a fan of The Beatles to know who they were and associate them with being English, but it would be a fairly easy leap for someone who was already inclined to like the Beatles to make.

Hence, Jules is a fan of the Beatles.

The "Hand Cannon" fired at Jules and Vincent was loaded with blanks
As noted elsewhere, some of the bullet holes in the wall are visible before the guy starts shooting. Set in Los Angeles, one of the roommates is an actor who took home a prop gun loaded with blanks.

The film is actually a prequel to the Marvel Cinematic Universe, specifically The Incredible Hulk
After the diner scene, Jules and Ringo both leave their life of crime. They are recruited into SHEILD and the Royal Marines respectively for their skills. Ringo receives the identity of Emil Blonsky, and eventually becomes Abomination, while Jules receives the identity of Nick Fury. Oh, and the briefcase contains the Tesseract.
 * Nick Fury doesn't age in the original Marvel universe. It is unsure wether he has this power as well in the Cinematic Universe, but if he did then this theory would be impossible.