Chrome Shelled Regios: The dites, which, for something that collapses into a perfect box the size of a remote control, can be anything from a pistol to a surfboard-sized BFS.
The Flash's entire costume is stored inside his ring, but the folding is aided with a special shrinking gas.
Iznogoud features an example of this. In "Iznogoud's Nightmarish Birthday", he opens a very small box and extracts a small piece of paper and proceeds to unfold it (it's so small that he needs a magnifying glass to start to unfold it). When completely unfolded, the paper is several metres large. Of course, the box had been given to him by the guild of mages...
At one point in the original comic-book version of The Tick, the Tick and Arthur take a road trip; the Tick insists that he knows how to fold up their (standard US-style) road map, but every time he tries, he produces an even larger wad of paper, until he's filled up the entire backseat of the car with it.
In Sorcerer's Apprentice, when Balthazar gives Dave the book, Dave marvels at how tiny and light it is. Balthazar then proceeds to unfold it several times until it's the size of an atlas, and hands it to Dave, who no longer finds it light. Balthazar then picks the top and lifts it and the book also becomes thick as a technical manual, nearly making Dave lose his balance.
In Transformers, the All Spark cube goes from the size of a house to the size of a basketball.
It also, apparently, forgoes the conservation of mass by allowing a teenager to lift it without trouble.
In The Simpsons Movie, Homer has a small billboard-sized poster of Alaska folded up to the size of a business card.
In Iron Man 2 the suitcase Iron Man suit. A full body powered armor suit all in a briefcase.
Glory Road. The foldbox which carries all of the team's equipment.
Rufo's baggage turned out to be a little black box about the size and shape of a portable typewriter. He opened it. And opened it again. And kept on opening it and kept right on unfolding its sides and letting them down until the durn thing was the size of a small moving van and even more packed.
Planetron, star of two children's books about the solar system and the galaxy, unfolds from the size of a toy robot to a fully functional spaceship, and back.
Live Action TV
Babylon 5: Minbari battle staff instantly extends from a small hand-held cylinder when you shake it.
Stargate SG-1: Jaffa helmets fold down into the collar after having glowing red eyes and apparently relatively thick armor. The Iris also folds up into a thin ring around the edge of the Stargate.
One Norse god can fold up a ship and stick it into his pocket.
Paranoia adventure The Yellow Clearance Black Box Blues, Mission 4. In order to enter Troubleshooter Headquarters the PCs must enter a huge machine. If they do so (or if they refuse to do so), the machine folds in on itself repeatedly until it's the size of a suitcase.
Dungeons & Dragons module I12 "Egg of the Phoenix". The title egg is an homage to the foldbox in the Glory Road entry in the Literature section. It can be opened up multiple times, each time becoming larger.
There is also a folding boat.
In 4th Ed., there's the "Compact Folding Astral Skiff", which folds to the size of a backpack and only weighs a few pounds, but folds out to a full-sized astral skiff which requires a crew of four. Of course, like many items in the book it's introduced in, it has some weird multidimensional physics applied to it.
The Pokémon games feature a bike that can be folded up small enough to fit into a backpack, although this may just be an example of Bag of Holding.
Marge buys Homer some large underwear for Christmas. After making sure it'll fit him by asking to two clerks to wear it together, one clerk then folds up the bed sheet sized underwear and stuffs into a gift box that would fit a wedding ring. He warns her to be careful when opening it.
In one episode of Back to the Future, Doc Brown builds a car that can fold itself into a briefcase, just like George Jetson's example above. However, it's still a car, and requires a crane to lift.
In one old episode of Family Guy, the Griffin family is preparing for the Y2K Apocalypse. Peter and Lois have this conversation...
Lois: What the hell, Peter?! You just ate a year's worth of dehydrated food!
Peter: Yeah, and it was a waste of money! I'm still hungry!
*Peter drinks a glass of water and his body immediately expands to several times its size, smashing the chair he's sitting on*
Peter: Everyone leave. I have to poop. NOW!!
The current world record for folding a piece of paper in half is 12 times. That's about 4,096 layers (2^12). It was done with an incredibly long "roll" of toilet paper, folded lengthwise. (For the record, the most you can get with notebook-style paper is 7, maybe 8 depending on folding technique. 8 folds is possible with tracing paper.) The MythBusters managed to get 11 folds in alternating directions, but this required a "sheet" of paper the size of a football field and the assistance of a forklift and a steamroller.
Vacuum packs kind of sort of do this with your clothes and other cloths since a good chunk of their volume is air.
"Astronaut Blankets", or metallic Mylar sheets, come in a pack no bigger than say two of those tissue packs stacked on their side, and can fold out to cover about a 8'x4' area. considering these are normally meant for campers, it's better if they're tiny.
On the other hand, any camper who has actually had a reason to open one has discovered the joys of trying to fold it back into that size (it's not possible, by the way, not without machines)
A teacher of Roald Dahl proved to his class that a sheet of paper 1/4" thick, folded 50 times in half, would reach from the Earth to the Sun.
Many engineers are working to create objects capable of this trick, working from the premise that much of the problem is in the empty space and unnecessary excess material present in most objectsnote for instance, thinner paper that is folded to be completely flat and smooth can be folded many more times than sloppily folded thicker paper. The holy grail is an object that functions like a dome tent- a bare-bones but strong and stable structure large enough to comfortably sleep two or more adults, which folds into a tiny bundle of tubes and cloth easily stored inside a hiker's pack. The mass remains constant, but unnecessary empty space is eliminated.
When folded, much like the tent, the object would be condensed into an ultra-compact form- ideally a nearly solid block or tube. When unfolded, however, a collapsible device could expand into a significantly larger structure utilizing Hoberman lattice structures and inflatable supports kept rigid by compressed air or a honeycomb structure, potentially covered in a thin layer of metal, plastic, or carbon fiber skin to give the illusion of a solid object. A well engineered device could be just as strong or stronger than an equivalent solid device, but much lighter. A shovel could fold to fit in a pocket, a self-deploying multi-wing modern surgical hospital in a single (rather heavy) shipping container, needing only the addition of perishable supplies, a water source, and staff.
Nanotech could take this even further by allowing the empty space to be minimized on a molecular level, condensing an entire structure into a solid ingot of material containing all the mass needed to extrude the full structure.