Superconductivity has a number of practical applications, such as Magnetic Resonance Imagine (MRI). However, its practical applications have been limited by the expense required to cool material to extremely low temperatures (usually approaching absolute zero) which have, so far, been required to reach superconductive states.
The editors of Superconductor Week track the stocks of seven companies which in their opinion "Superconductor-related technologies are a core business dimension for the companies"  The seven Superconductor Week tracked companies are:
The inclusion of Siemens is misleading, because Siemens is the equivalent of a US Fortune 100 company, and therefore while the company's business employs superconducting technology in some areas, superconductivity makes up a small percentage of the company's overall business.
Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes, who was studying the resistance of solid mercury at cryogenic temperatures using the recently-discovered liquid helium as a refrigerant. At the temperature of 4.2 K, he observed that the resistance abruptly disappeared. In subsequent decades, superconductivity was found in several other materials. In 1913, lead was found to superconduct at 7 K, and in 1941 niobium nitride was found to superconduct at 16 K. 
The complete microscopic theory of superconductivity was finally proposed in 1957 by Bardeen, Cooper, and Schrieffer. Independently, the superconductivity phenomenon was explained by Nikolay Bogolyubov. This BCS theory explained the superconducting current as a superfluid of Cooper pairs, pairs of electrons interacting through the exchange of phonons. For this work, the authors were awarded the Nobel Prize in 1972. 
In 1962, the first commercial superconducting wire, a niobium-titanium alloy, was developed by researchers at Westinghouse. In the same year, Josephson made the important theoretical prediction that a supercurrent can flow between two pieces of superconductor separated by a thin layer of insulator. This phenomenon, now called the Josephson effect, is exploited by superconducting devices such as SQUIDs.
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Josephson was awarded the Nobel Prize for this work in 1973. 
Until 1986, physicists had believed that BCS theory forbade superconductivity at temperatures above about 30 K. In that year, Bednorz and Müller discovered superconductivity in a lanthanum-based cuprate perovskite material, which had a transition temperature of 35 K (Nobel Prize in Physics, 1987). It was shortly found by Paul C. W. Chu of the University of Houston and M.K. Wu at the University of Alabama in Huntsville that replacing the lanthanum with yttrium, i.e. making YBCO, raised the critical temperature to 92 K, which was important because liquid nitrogen could then be used as a refrigerant (at atmospheric pressure, the boiling point of nitrogen is 77 K.) This is important commercially because liquid nitrogen can be produced cheaply on-site with no raw materials, and is not prone to some of the problems (solid air plugs, et cetera) of helium in piping. 
As of October 2007, the highest temperature superconductor is a ceramic material consisting of thallium, mercury, copper, barium, calcium, and oxygen, with Tc=138 K 
More recently in January 2008 there was a report of Tc=181 K and in March 2008 a Tc=185 K. 
The first company to capitalize on high-temperature superconductors was Illinois Superconductor (today known as ISCO International), formed in 1989. This amalgam of government, private-industry and academic interests introduced a depth sensor for medical equipment that was able to operate at liquid nitrogen temperatures (~ 77K). 
By impinging a strong superconductor-derived magnetic field into the body, hydrogen atoms that exist in the body's water and fat molecules are forced to accept energy from the magnetic field. They then release this energy at a frequency that can be detected and displayed graphically by a computer. Magnetic Resonance Imaging (MRI) was actually discovered in the mid 1940's. But, the first MRI exam on a human being was not performed until July 3, 1977. And, it took almost five hours to produce one image! Today's faster computers process the data in much less time. 
Electric generators made with superconducting wire are far more efficient than conventional generators wound with copper wire. In fact, their efficiency is above 99% and their size about half that of conventional generators. * * * General Electric has estimated the potential worldwide market for superconducting generators in the next decade at around $20-30 billion dollars. Late in 2002 GE Power Systems received $12.3 million in funding from the U.S. Department of Energy to move high-temperature superconducting generator technology toward full commercialization.
According to the US Department of Energy release dated 5/28/2004, Superconductor Partnership for Electric Systems Generator "partnership was awarded in late 2002. A 1.5 MVA demonstration machine was built and tested successfully. Conceptual designs for 100 and 250 MVA generators were developed, and a preliminary design for a 100 MVA rotor is largely complete. Components of the rotor are now being fabricated and tested." The project goal was to "Develop and test a 100 MVA prototype High Temperature Superconducting generator." 
Other commercial power projects in the works that employ superconductor technology include energy storage to enhance power stability.
An idealized application for superconductors is to employ them in the transmission of commercial power to cities. However, due to the high cost and impracticality of cooling miles of superconducting wire to cryogenic temperatures, this has only happened with short "test runs" 
Superconductors have also found widespread applications in the military. HTSC [high temperature super conductor] SQUIDS are being used by the U.S. NAVY to detect mines and submarines. And, significantly smaller motors are being built for NAVY ships using superconducting wire and "tape". In mid-July, 2001, American Superconductor unveiled a 5000-horsepower motor made with superconducting wire (below). An even larger 36.5 MW HTS ship propulsion motor was delivered to the U.S. Navy in late 2006.
The newest application for HTS [high temperature superconductor] wire is in the degaussing of naval vessels. American Superconductor has announced the development of a superconducting degaussing cable. Degaussing of a ship's hull eliminates residual magnetic fields which might otherwise give away a ship's presence. In addition to reduced power requirements, HTS degaussing cable offers reduced size and weight.
The military is also looking at using superconductive tape as a means of reducing the length of very low frequency antennas employed on submarines. Normally, the lower the frequency, the longer an antenna must be. However, inserting a coil of wire ahead of the antenna will make it function as if it were much longer. Unfortunately, this loading coil also increases system losses by adding the resistance in the coil's wire. Using superconductive materials can significantly reduce losses in this coil. The Electronic Materials and Devices Research Group at University of Birmingham (UK) is credited with creating the first superconducting microwave antenna. Applications engineers suggest that superconducting carbon nanotubes might be an ideal nano-antenna for high-gigahertz and terahertz frequencies, once a method of achieving zero "on tube" contact resistance is perfected.
The most ignominious military use of superconductors may come with the deployment of "E-bombs". These are devices that make use of strong, superconductor-derived magnetic fields to create a fast, high-intensity electro-magnetic pulse (EMP) to disable an enemy's electronic equipment. Such a device saw its first use in wartime in March 2003 when US Forces attacked an Iraqi broadcast facility. 
"Superconductivity is a state in which a material experiences no resistance to electricity. This lack of resistance means that almost no electricity is lost to heat when a direct current is passed through a superconductor. Superconductors can thus generate very strong magnetic fields without the heat generation and electric current losses to resistance that occur with conventional conductors."