Mushrooms have always occupied a curious spot in the human psyche. Associated with magic, spiritual power and mystical revelation, mushrooms have been used as foods, healing agents, poisons, ritual artifacts, and hallucinogens. They are hunted by enthusiasts the world over, and play a central role in numerous lavish festivals and museum displays.
Mushrooms comprise a category of nutritional products that have been gaining scientific and medical attention these days.  A rapidly expanding body of research worldwide is finding value in the use of mushrooms for the prevention and treatment of health problems - ranging from immune disorders, viral diseases, high cholesterol, coronary disease, liver disease, and cancer. Three particular mushrooms of medicinal value are listed below.
 
Shiitake, Lentinus edodes
On the medicinal side of things, shiitake is a heavyweight mushroom that fights many common health problems. The mushroom contains a polysaccharide called lentinan, and another known as Beta 1.3 glucan, both of which appear to possess powerful anti-viral and anti-tumor powers.
Shiitake also enhances immunity by boosting the production of T cells, macrophages, phagocytes, natural killer cells and other white blood cells. It even helps to fight flu, due to the presence of a polysaccharide known as KS-2. Further clinical studies show that Shiitake even protects against the damaging effects of radiation and chemotherapy.
Nutritionally, shiitake is very heart healthy and has been shown to lower cholesterol. Studies conducted at the National Institute of Nutrition demonstrated that consumption of shiitake leads to a drop in serum cholesterol of between 7 and 12 percent. Russian and U.S. studies show that shiitake inhibits platelet aggregation in the blood, thereby reducing the buildup of arterial plaque - which is the cause of hardened arteries.  This mushroom also inhibits the formation of arterial lesions, and lowers high blood pressure.
 
Reishi, Ganoderma lucidum
One of the most revered tonic herbs in traditional Chinese medicine, reishi is purported not only to enhance longevity, but to imbue the mushroom eater with spiritual radiance and wisdom. Reishi is presumed to uplift and transform the mind by expunging psychic waste, thereby helping to facilitate enlightenment.
Spiritual realization aside, the mushroom's medicinal properties, however, do explain the high regard in which it is held.  Studies conducted around the world show that Reishi mushrooms fight tumors, presumably due to the presence of the polysaccharide Beta 1.3, which occurs in shiitake. Reishi also increases immune-protective T cells and macrophages. The mushroom demonstrates anti-bacterial and anti-viral activity, and accelerates recovery time from infectious Hepatitis.
Reishi protects the liver from damage  by chemical toxins such as carbon tetrachloride. The presence of cyclooctasulphur, a histamine-inhibiting compound in reishi, makes this  mushroom helpful in the treatment of chronic bronchitis. Reishi is also a valuable source of adenosine, which inhibits platelet aggregation - a contributor to arterial plaque -  thus providing significant heart- protective activity.

 Maitake, Grifola frondosa

 Though this mushroom doesn't carry the same distinguished traditional reputation of shiitake and reishi, it may be valuable in the fight against HIV. At an International Conference on AIDS in Amsterdam, maitake was reported to inhibit the HIV virus in vitro. While the mushroom doesn’t kill the HIV virus in humans, it may help as part of a total integrated medicine program. Some Japanese research even suggests that maitake may provide stronger immune-enhancing properties than the previous mushrooms listed.
Like both reishi and shiitake, maitake contains a wide array of valuable sterols, glycosides, amino acids, enzymes, polysaccharides, vitamins, minerals and other medicinal compounds. Research shows that the compounds in maitake mushrooms (including Beta 1.3 glucan again) fight breast cancer cells and skin cancer cells. These compounds appear to bolster the immune system by stimulating the production of macrophages, killer T-cells, T lymphocytes, and natural killer cells. Maitake mushrooms also reduce high blood pressure, and help to stabilize blood sugar levels.  
What’s the best way to take these mushrooms? Eating them as foods is surely the way to go, but you may have a hard time finding fresh or dried maitake or reishi mushrooms at a conventional grocery store.  Chinese groceries or traditional Chinese apothecaries typically carry these exotic foods, so if you do find any of these three mushrooms fresh or dried, buy them by the bag and learn to cook them in various dishes.
You can also find mushroom supplements on the market, and they vary widely.  Some companies provide special extracts of the medicinal mushrooms described above.