Radon's one practical use has been in medicine. A few hospitals produced it by pumping it from a radium source and sealing it in minute tubes, called seeds or needles, for application to patients. One gram of radium makes only about 0.0001 milliliter of radon a day. The practice has been largely discontinued: hospitals can get seeds directly from suppliers made up for the day of use, and other radioisotopes, such as cobalt-60, are now used in place of radium.
Its other use is in research. Radon-220 and radon-222 help scientists study how gases move underground and in the atmosphere, and because radon is lost quickly from open water, a stretch of stream or river with a high radon concentration points to groundwater flowing in. Radon-222 has also been used to date groundwater.
Radon has no use as an ingredient in everyday products: it is essentially inert, and its most stable isotope lasts only days. In homes it is something to keep out, since exposure to radon is the second leading cause of lung cancer after smoking.
What to do next: read the radon element guide for its properties and history, and if you want to know the level in your own home, test the lowest lived-in level and read the result with the interpreter.