This topic is under construction.. I'm leaving it up for now, but it might get a little messy. Thanks for your patience!
Introduction
So you've done a bit of research, and you've decided that a hamster might be a good pet for you. Congratulations! 
You're about to enter the wonderful world of hamster keeping. As you begin your in-depth research, one of the first questions you’re probably asking yourself is what type of cage you should keep it in. Obviously, hamsters need to be kept in cages. However, we’ve taken to modifying the word “cage” to mean something a little more positive:
Contained Area for Great Exercise
Contained Area - A good cage should be secure and safe, with no obvious escape routes. This includes having a lid - while some cages can be okay without lids, this is generally not recommended unless the cage is extremely deep (over 36" of height).
Great Exercise - A good cage will provide lots of floor space for a hamster to exercise. To figure out floor space, simply measure the length and width of the bottom inside of the base of the cage. Multiple those two numbers and that gives you floor space! Note that you should never use the dimensions on the box / label to evaluate floor space - those are often outer dimensions, and will almost always give the impression that the cage is much larger than it actually is. The cage must also be able to accommodate at least 6" of bedding for burrowing.
General Information about Hamster Cages
Hamsters need far, far larger cages than most people realize.
Species Differences (re: if you are considering a Syrian hamster)
It is generally understood that Syrian hamsters need much larger cages than the other species. This is not only due to their larger size, but their habits as well. Syrians simply tend to get bored and restless easier. Many Syrian keepers will attest that they've had hamsters express boredom in even the largest cages on this list. If you are intending to get a Syrian, the best option is to err on the side of getting a very large cage, lest you have to spend a ton of money upgrading over and over again.
Do-It-Yourself
HoppingHammy’s DIY cage (modified table frame)
DIY cages are the most flexible option available for hamster cages. They are becoming increasingly common, as they are far more customizable then commercial options for a pretty reasonable price. DIY cages can either be furniture “hacks” (such as a cage made out of a bookshelf or wardrobe) or a cage made completely from scratch.
DIY cages can easily be among the biggest and best cages, however, they do require the most amount of work.
Large Tanks
HoppingHammy’s 55-gallon tank
Yes, hamsters can be housed in fish and reptile tanks! And in fact, tanks are some of the best homes for hamsters out there. Tanks have the great benefit of requiring no assembly – they’re super-easy to set up. When used with a properly-fitted lid, they are secure, sturdy, and virtually escape proof.
Tanks are easy to find in many pet stores. If you can't find one to your liking, you may wish to seek out an aquarium supply shop, as they will likely have more choices.
Larger tanks than the ones listed are certainly fine as well! If you have the space and money for a massive 100+ gallon aquarium, all the better! Your hamster will certainly appreciate the extra space!
Bin Cages
Taxonomist’s Christmas Tree Bin (lid not pictured)
Bin cages are a do-it-yourself option. A bin cage is a plastic storage tub modified with mesh windows (typically on the lid) for ventilation. Bin cages are a budget option - they are inexpensive, lightweight, and easy to transport and clean. You can read more about bin cages by clicking here.
They do require some construction, and tools will be needed to build one. Wire mesh (hardware cloth) can be purchased at most home improvement stores. A sharp knife or Dremel will be needed to cut the plastic, and a drill will be useful for drilling holes to attach the mesh. Zip ties are screws can be used to secure the mesh to the plastic.
One downside to bin cages is that they do tend to be on the small side. However, it is possible (and strongly recommended) to connect multiple bin cages using large-diameter PVC pipes in order to provide more space. Realistically, the IRIS bin listed first below is probably the only bin cage that would be considered a great cage on its own.
Bin cage options:
IRIS Christmas Tree Box / Container Store 44 Gallon Tote
47" long x 17" wide x 14" tall
Sterilite 116 Quart Ultra Latch Box
30" long x 17.5" wide x 13.5" tall
Sterilite Modular Stacker 50 Gallon / 189L
34" long x 17.5" wide x 17.875" tall
IKEA Detolf
Taxonomist’s Detolf (lid not pictured)
The Detolf is a particular variety of not-quite-DIY cage. The Detolf is a tall glass display cabinet with a wooden top and base. However, when laid on its back and assembled without shelves or the door, it ends up being functionally identical to a large tank. It is quite reasonably priced for its size.
A Detolf does require assembly, and is best put together with at least 2 people. A DIY lid will need to be made, as the Detolf is shallow and easily escapable without one.
IKEA Detolf
61.5" long x 15.5" wide x 13.625" tall
Living World Green Eco Wooden Habitat (Large Size Only)
[Direct image not currently available. Images can be found on ZooPlus website]
This cage is probably the best ready-made option available. It is hamster-safe from the get-go, though it may be a good idea to line the base with plexiglass or something similar to prevent chewing.
Its only downfall is its price. This cage is expensive, especially for those in the US. It is available for $300 (including shipping) on ebay. If you choose this cage, get the largest size. The smaller sizes are simply not worth the price - they're significantly smaller, but only slightly less expensive. The largest size is easily the best value.
Living World Green Eco Wooden Habitat (Large)
46.6" long x 30.9" wide x 24" tall
Large Rabbit and Guinea Pig Cages
A display guinea pig cage (picture taken at a pet store by Taxonomist)
Note: This cage, being a display, is obviously shown without the wire mesh that would be needed to make it hamster-safe! It is also probably not the best size, but is included only to show the general appearance of these types of cages.
While many hamster cages are indeed too small to house a hamster, some rabbit and guinea pig cages can actually make good homes for hamsters. The one "catch" is that the bar spacing on these cages is often too wide - while a rabbit or guinea pig could never get out, a tiny hamster could easily squeeze between the bars. Because of these, these cages need to have a layer of wire mesh added on top of the bars in order to close off the gaps. Be warned before attempting this cage type that this is notoriously tricky to accomplish.
Also, make sure to remove any wire grates on the floor of the habitat before use!
Here are just a few options for this type of cage. There are obviously many other choices. When looking at these types of cages, make sure that the advertised dimensions are at least 40" long x 15" wide. Because of the way the official on-the-box dimensions are taken (largest outside part of the cage), the actual living space at the base of the cage is much smaller than the given dimensions.
(As a note, this is also why the dimensions are not available for these cages - it's impossible to measure them without owning them or at least seeing them out of the box.)
Hagen Living World Deluxe Habitat, XL
[dimensions unavailable at this time]
National Geographic Connectible Rabbit Small Animal Habitat
[dimensions unavailable at this time]
You & Me Living the Dream Small Animal Home
[dimensions unavailable at this time]
Edited by Taxonomist, 13 April 2019 - 08:23 AM.
The reason is that we're trying to take the focus off of numbers and onto "good cages." That was part of what went downhill with the previous minimum cage size - everyone got so fixated on a particular number that the message itself got lost. We don't want that to happen again.
