Vitamin b3, or nicotinic acid. A water soluble vitamin required by all living things.

Chemically, niacin is a heterocyclic ring connected to a carboxyl group. Another form is niacinamide, in which the carboxyl group is replaced with an amide group. Despite the similarity in name and structure between nicotinic acid and nicotine, they have very few similarities in function.

When a person does not get enough niacin in their diet, (or alternatively, enough tryptophan to synthesize it from), they develop a syndrome called Pellagra, characterized by dementia, dermatitis, and diarrhea. Full blow Pellagra is rare in most of the world, but sometimes shows up in parts of the world with unbalanced nutrition.

People who take niacin should expect a somewhat unpleasant reaction of flushing and itching the first few times they take niacin. This is mostly harmless.

Niacin is used medicinally to lower cholesterol levels. Some doctors, such as Abram Hoffer and Linus Pauling believe that large doses of niacin can increase over all health.

The Scientologists think that niacin is effective in combatting drug addiction, among other things, and although they may be correct, I don't know if their reasons for believing this are.

Personally, I have noticed that niacin gives me a pleasently spacey, hypnotized feeling, although most people who I have convinced to try it don't report the same thing.
Addendum: Niacin also gives me vivid dreams that border on lucid. Last night, after taking 200 milligrams of Niacin together with 9 milligrams of Melatonin, I had a quite nice lucid dream, where I managed to teleport around.