Liquid nitrogen and liquid helium In cryogenics, liquid nitrogen and liquid helium are often used as coolants. Other than their low boiling point of 77K and 4.2K respectively, what properties make them suitable  as coolants? Are there any other possible candidates around the same temperature range?
 A: Liquid nitrogen is used because nitrogen is extremely abundant on earth. Nitrogen makes up approximately 78% of the atmosphere by volume. Hence, liquid nitrogen is rather easy to make (and consequently cheap). I've heard for instance that Fermilab buys liquid nitrogen for cheaper than what you pay for water.
Liquid helium is useful for things that must go to much lower temperatures than 77K. It remains liquid (though it does change into a superfluid) even at absolute zero at atmospheric pressure, while essentially everything else is a solid at those temperatures. Solids aren't useful as coolants for obvious reasons, so liquid helium really is the only option.
There are other cold liquids that could be used, like liquid oxygen (BP at 90K) or hydrogen (BP at 23K), but these don't offer many advantages over nitrogen and helium. They are more reactive as well, which may or may not be a concern depending on what it is you are cooling. For most things nitrogen is sufficient, and for most things it doesn't work for helium does, so that's why they're almost universal.
A: I feel it is worth to emphasize over @logan's answer that both helium and nitrogen are inert and non-flammable. That makes them as safe to use as could be (the only dangers coming from the cryogenic properties itself, i.e. suffocation, explosion, and frostbite.) In contrast, liquid hydrogen or oxygen are extremely dangerous due to their high reactivity.
Note that liquid helium has a low heat capacity, and in that sense is not ideal, but still the coolant of choice for temperatures of tens of kelvin and below due to a lack of alternatives.
Note also that often, liquid cryogens are used in closed cycles. Such cryocoolers conveniently provide cold with the flip of a switch.
Finally, both helium isotopes He3 and He4 are being used, particularly in dilution refrigerators, when temperatures below 1K are required.
