Garlic

Garlic has been used for centuries (perhaps millennia) as both a tasty, healthy food and a medicine. Garlic is part of the Allium family (which includes onions, leeks, shallots and chives as well) and has many of the same benefits - but garlic is more beneficial (and stronger-tasting) than any of the others.
Garlic contains many sulphur compounds, which give it its taste and smell. Most of these compounds are very unstable, but the main one that most of the others come from is called allicin (from the Latin for garlic), which is made from another compound called alliin, and that is reasonably stable until the garlic is crushed or cut. When that is done, the alliin is acted upon by an enzyme called alliinase to produce allicin.
Garlic has many health benefits. It has a strong effect on cholesterol levels, particularly the "bad" cholesterol LDL. Garlic has a very strong antimicrobial effect, on just about all microbes including fungi, protozoa, viruses and bacteria - in fact, garlic has been used as recently as World War I, to help stop battle injuries from festering.
Garlic also reduces mucus buildup, and hence helps in colds and flu, and the sinusitis and bronchitis that sometimes result from them. Garlic also helps increase the fluidity of mucus, and therefore helps to get rid of catarrh.
Garlic, because of high levels of sulphur-containing amino acids, also helps detoxify heavy metals such as lead, mercury and cadmium, by binding onto them and flushing them out of the system.
Garlic in supplement form comes essentially in three forms. The closest to eating garlic is freeze-dried; this is made from intact garlic bulbs and then crushed into powder. This has the advantage of keeping the garlic in a state as close as possible to the original plant, but also keeps the disadvantages - the smell, which some people dislike, and its mildly irritant qualities. Next is aged garlic extract, which is made by keeping mashed garlic in tanks cooled to just above freezing, for many months. This product is probably the best at cholesterol reduction and heavy metal reduction, but is not as strong as either of the other two in killing bugs. Finally, there is garlic oil, which is made by steam-distilling mashed garlic. This is probably the strongest antimicrobial among garlic supplements, but is extremely pungent.