Also known as Ve-Tsin, "accent" or MSG. Often added to food as a 'taste enhancer'.
The Chinese consider that there are five fundamental tastes: salt, sweet, sour, bitter and 'savory'. The Japanese call this taste umami (OO-mommy). MSG would be the defining example of savory, in the same way that sugar is the defining example of sweet.
Apparently, a Japanese scientist demonstrated that we have taste buds for glutamate. However, the existence of umami is by no means universally accepted.
As the article Making Sense of Taste from Scientific American - www.sciam.com 
- states::
Researchers are also studying a receptor that might be responsible for a taste Japanese scientists call umami, which loosely translates into "meaty" or "savory." In 1998 Nirupa Chaudhari and Stephen D. Roper of the University of Miami isolated a receptor from rat tissue that binds to the amino acid glutamate and proposed that it underlies the umami taste. Other researchers, however, are still skeptical that umami constitutes a fifth major taste as significant as sweet, sour, salty and bitter. Although the taste of glutamate might be a unique sensation, only the Japanese have a word for it.
Apparently some people can be allergic or sensitive to MSG, causing migraines and other discomfort. This has led to wide-spread rumours (compare: Nutra Sweet, Dihydrogen Monoxide) about MSG causing something called the 'China Restaurant Syndrome'. We have sodium ions and glutamate ions in us at all times, so it is not known whether MSG really is dangerous, and if so, why.
As this page originally read::
The salt of Glutamic acid is a white crystal with a very harsh rancid metallic taste that causes migraine headaches, irritability, optical flashes, trembling, and death. So, of course, they put it on nearly all processed foods.
Please see www.truthinlabeling.org for more info on this.
And now for something Completely Different:
The council of a small town called upon the services of a noted herpetologist to study the decline in numbers of the amphibian population at the local pond. The herpetologist arrived during breeding season, and immediately noticed that the males were slipping off the females whenever they attempted to mount. He whipped up a concoction to add to the waters of the pond that contained two parts of this and three parts of that, but most critically, exaclty one part of sodium. The chairman of the council groaned at the inevitable punchline that was about to be delivered: "Oh no! Surely you don't mean...?" "That's correct," replied the herpetologist. "They need a monosodium glue to mate."
See also: Worst Pun Ever
See original on c2.com