It's brown, it lives in the sea, and eating it is as nutritious as drinking human breast milk. Fucoidan is a sulfated polysaccharide found in seaweeds such as kombu, limu moui, bladderwrack, wakame, mozuku, and hijiki. The brown seaweeds typically consist of about 4% fucoidan.

"Fucoidan is like a closely guarded secret, as an antioxidant it has certainly piqued the interest of many scientists. Fucoidan has been scientifically proven to enhance immunity and cellular health to the point where it helps heal wounds, supports normal blood pressure, detoxifies the body against heavy metals, helps prevent the common colds and flu as well as, yes fight Cancer. There I said it. Fight Cancer." (Ezine)
A cure to cancer?
The National Library of Medicine's research from over 700 studies indicates that fucoidan demonstrates great potential as a safe nutritional answer for a wide variety of health complaints. There are two types of fucoidan, F-Fucoidan, and U-Fucoidan. "The first type, bearing the name F-Fucoidan, consists mainly of sulfated fucose. The second type bears the name U-Fucoidan, and approximately 20 percent of it consists of glucuronic acid." (Naturodoc) The Biomedical Research Laboratories of Takara Shuzo and the Research Institute for Glycotechnology Advancement have indiciations that U-Fucoidan causes cancer cells to self-destruct.
Numerous accounts have ascribed to Fucoidan properties such as the ability to act as an anti-contraceptive, to reduce cholesterol levels, and to act as an anti-tumor agent. However, a definitive consensus concerning the precise nature of Fucoidan has still not been reached. (Naturodoc)

Japanese heritage
"From ancient times (dating from the Jomon era, approximately before the 2nd Century BC onwards), brown seaweed has been a mainstay of the traditional Japanese diet." (Naturodoc) There is also statistical support that U-Fucoidan is a preventer of cancer, as the inhabitants of Okinawa, have not only the highest life expectancies in Japan, but also the lowest cancer death rate. Other than taking fucoidan, I'm sure the Japanese would have or still would benefit from alginate, which is a natural absorbent of radioactive elements. (Fucoidan Net) That is particularly interesting considering the atom bomb was dropped in Japan, and alginate may have been a helpful remedy to the radioactive poisoning.

Fucoidan is also a good way to slow down blood clotting. (Mskcc)

You can take fucoidan in seaweed form, as a pill, or in a drink. I drink eXfuze which has fucoidan and six other antioxidants in it.

"Imagine a natural immunity boost ingredient that would have no harmful side effects, would have multiple health benefits, would come from a renewable natural product, would be inexpensive, and would reduce the incidence of cancer, common cold, respiratory and gastrointestinal infections. Fucoidan possess all of these ideal properties." (Ezine)

Sources:
http://www.naturodoc.com/library/detox/U-Fucoidan.htm
http://ezinearticles.com/?Fucoidan---Is-There-a-Natural-Cure-for-Cancer?&id=382978
www.mskcc.org/mskcc/html/69227.cfm
http://www.eXfuze.com