Helium

Helium (also known as "He" in stoichiometry circles) is the lightest of the noble gases. It holds court in stars (named after the Greek god of the sun, Helios) and kid's balloons. Pierre-Jules-César Janssen discovered it in 1868 while studying the spectral lines of the sun.

Helium is the second lightest element, heavier only than hydrogen, and has an atomic number of 2 and an atomic weight of 4.003. Due to its non-reactive nature, especially compared to the extremely flammable hydrogen, helium is the preferred element whenever a light gas is needed. This is why it fills kiddie balloons and blimps.

Helium is also the second most abundant element in the universe (about 8% of all atoms), being one of the few elements created from the Big Bang. It is the byproduct of nuclear fusion in stars when hydrogen atoms are fused together. Helium has extremely low melting and boiling points (0.95K and 4.22K, respectively) which is why it's more often than not in gaseous form. Its low boiling point makes it ideal for manufacturing processes that require extremely low temperatures, such as making MRI magnets, fiber optic cables, and LCDs. Its lightness means helium easily escapes into space; it makes up only 0.0005% of the Earth's atmosphere.

Breathing it
Due to its odd displacement qualities, it has peculiar effects on human vocal cords at normal pressures, and inhalation of it will make you sound like Alvin and the Chipmunks. However, at extremely high pressures (like near the bottom of the sea), it's been found to be extremely useful. Deep-sea divers breathing pressurized air have problems with bubbles of nitrogen working into their joints, which tend to painfully come out if they depressurize too quickly, a condition called the "bends". (Breathing pure pressurized oxygen can cause even worse problems.) While normal dehydrated air is used for typical scuba diving, two mixtures involving helium are used at greater depths:
 * Heliox, a mixture of just helium and oxygen. This is useful down to depths of about 130 meters, but deeper dives using heliox can result in High Pressure Nervous Syndrome.  So at greater depths, you need:
 * Tri-Mix, a mixture of helium, oxygen, and nitrogen. Due to nitrogen being cheaper than helium, tri-mix is now the preferred deep-dive breathing mixture at all depths.

Nuclear fusion
Helium-3, a rare but stable isotope of helium with only one neutron in its nucleus, has shown some promise for use as a fuel in nuclear fusion reactions. No man-made helium-3 fusion process has yet exceeded the break-even threshold, though, and even if such a process were to be invented the supply of helium-3 on Earth is extremely small.

Certain enterprising individuals have suggested mining operations on the moon to harvest the helium-3 in the lunar regolith, which should be more abundant than helium-3 on Earth. ("More abundant" doesn't mean cheap and plentiful, though; it's still rare even on the moon.) However, these suggestions often stem not so much from an interest in helium-3 fusion, as from a desire to have some excuse to go to the moon now that the Apollo program has shut down.

Helium shortage
Since helium is so light and does not bind with other elements, none from the initial formation of the planet remains. All existing helium, therefore, is relatively new, manufactured by the alpha decay of radioactive elements deep inside the planet.

Unfortunately, the geological alpha decay process doesn't produce helium anywhere near as fast as we humans are using it. The U.S. national helium reserve is expected to run out of helium as early as 2018. This is apparently due to a 1996 congressional mandate requiring the U.S.'s entire helium reserve to be sold off by 2025 -- if the market alone were to set the price, a kid's helium balloon would cost about a hundred dollars. Fortunately, the Responsible Helium Administration and Stewardship Act, passed near-unanimously in 2013 at the beginning of May, will help to stave off this unpleasant future, but the National Helium Reserve will still run dry within a decade of its passage.