The most common coolant used in real life is by far the regular water. It is easily accessible, it is abundand, you can get it pretty much everywhere and dump it back as soon as you don't need it anymore with no negative consequences. It has only one shortcoming: it freezes solid at 0 °C (32 °F, 273.15 K). If you want to go below the water freezing point, you need to come up with something else. This "something else" would usually be an antifreeze based on hydrocarbon solutions, which will get you to around -60 °C (-76 °F, 213 K), which is usually enough for things like engine or machinery cooling, unless you want to go to antarctica. But if you want to go to the extreme cold temperatures, your "something else" would almost always be the liquid nitrogen.
Liquid nitrogen is a nitrogen in a liquid state. As the basic physics course tought us, any matter can exist in three different states. While this is not exactly true for everything, this fully applies to the air that surrounds us, which can be made liquid if one brings the temperature low enough. The air consists primarily of nitrogen (78%) and oxygen (21%), but since the boiling point of the nitrogen is higher than that of the oxygen, if you start cooling down the air, the liquid nitrogen would be the first thing you'd get note .
Because of this the liquid nitrogen is the Flash Freezing Coolant used both in media and in real life. The only reason you'd be using something else is if you need to go even lower than 77 K (−196 °C, −321 °F), which is the nitrogen's boiling pointnote . And yes, it is still called "boiling point" even though we approach it from the "other side": instead of heating the liquid until it becomes gas like you'd do with liquid water turning it into steam, we cool down the gas until it condenses into a liquid. Still, it is a boiling point, and the liquid nitrogen that you may encounter both in real life and in the movies would be boiling, which is why its display always comes with lots of fog and bubbles.
That being said, the liquid nitrogen in real live behaves quite differently from the usual Flash Freezing Coolant you'd see in movies. First and foremost: it won't flash-freeze anything. And it will even struggle at regular freezing at all. There is a physical reason for this: in order to turn liquid into gas you need to provide it with the additional heat, which won't increase the temperature, and will be only used for evaporation. This is called the "heat of vaporization" and in a broad sence, this reflects how much heat the potential coolant can take from the object it cools. As soon as the coolant evaporates, it stops behaving like a coolant, and starts behaving like an insulator, since as a gas it has very low thermal conductivity. And the heat of vaporization for the liquid nitrogen is just 0,2 MJ/kg. For comparison, water has 2,26 MJ/kg. Liquid nitrogen is eleven times less effective at cooling stuff than water. So much so, that you can even submerge your hand in it for a short duration of time (about a second) with no ill effect. During the viral "ice bucket challenge" of 2014 there were several people who poured liquid nitrogen on themselves with no consequences.
Needless to say, Don't Try This at Home, as while liquid nitrogen won't turn you into a Human Popsicle, it is still quite hazardous, and can hurt you in different ways. If you expose your skin to it, you will get frostbite eventually. And the biggest problem is: you won't even feel anything until it's too late. The skin cells that detect cold are located very close to the surface, and because of the extreme temperature of the liquid nitrogen, they get frozen before they have the time to signal the brains about it. When your skin starts to thaw after the exposure - that's when you'd feel pain, but that would already be too late.
But the most dangerous thing that could happen because of the liquid nitrogen is asphyxiation. You would never guess it from the movies, but death by asphyxiation is by far the biggest hazard when dealing with it. When you release a large amount of nitrogen in a confined space, you will reduce the amount of oxygen there, and unfortunately, humans (or other animals) cannot detect the lack of oxygen through their natural senses. We can only smell if something harmful is present in the air, not when something is absent from it. The only symptom you'll get is that you'd feel really sleepy. And by that point you probably have less than a minute of useful consciousness left to run for the open air as fast as possible. If you have a 5-liter dewar
full of liquid nitrogen, when evaporated, it'll take 3-4 cubic meters as a gas, which is easily enough to bring the partial oxygen pressure below the non-survivable level in a small room.
Still, if you take the necessary precautions and know what you are doing, dealing with the liquid nitrogen is not much more dangerous than dealing with the liquid natural gas. It is not a restricted or controlled substance in any jurisdiction, and you don't need any licenses to store or even produce it. Yes, this is not a secret resource from the goverment laboratories, you can freely buy a liquid nitrogen generator for a price of a (somewhat expensive) car, and produce as much of it as your electricity bills allow. Or, if you need just a little of it for a personal project, find the nearest university and ask people there. Even if they do not have a cryogenic laboratory on site, they surely know where there is one. Just come with your own dewar. Unlike liquid nitrogen, which you literally make out of thin air, dewars are expensive.