Is there anything colder than liquid nitrogen?
Yes, there are things colder than liquid nitrogen, like most of the Universe! I assume, though, that you mean things on the Earth. There actually is an entire branch of science called cryogenics that deals with really cold things. Generally the science of cryogenics is when the temperature goes below that which we can reach with conventional refrigeration equipment, around 250 degrees (Fahrenheit) below zero. Many gases are liquid at these low temperatures. They can be colder, but the following list is the temperature at which these gases boil. Before their temperature can get any higher all the liquid must boil away and turn back into a gas.
On Earth, none of these gases naturally exist as a liquid. They are only liquids when someone has made them into a liquid by artificially cooling them. We just live on too toasty of a planet. There are places, here in our solar system, though, where these materials not only exist as a liquid, in some places they are solids. How would you like to see air snow? I guess you could redefine what an airball was.
I was not joking when I said the entire universe is colder than liquid nitrogen. Many years ago a group of scientist, who were trying to figure out why radios had static (noise), discovered that they had accidentally measured the average temperature of the universe. It turned out to be minus 269 degrees Celsius or minus 452 degrees Fahrenheit.
We use liquid nitrogen, but mostly liquid helium, here at Jefferson Lab to cool our electron accelerator. It allows us to save enormous amount of energy and money. Liquid nitrogen is a LOT cheaper than the helium so we use that for our demonstrations. Even though Helium is the second most common element in the universe there is only a fixed amount available on the earth, so we don't waste any of it. Liquid nitrogen is very inexpensive because we can get it from air. Liquid oxygen is also extracted from air, but for each gallon of liquid oxygen you get three of liquid nitrogen. There are many industrial uses of oxygen, like in the manufacturing of steel, so there is usually a surplus of liquid nitrogen.
Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)