In one episode of the TV show "Top Chef", the contestants were given the theme of umami to cook for. All of them somehow made a mad dash for mushrooms.
What exactly is umami? The so-called fifth taste, it is the perception of savoriness or meatiness in a taste. The key item being detected is the amino acid glutamate, which also acts as a neurotransmitter. Thus, the creation of the flavoring agent monosodium glutamate (MSG) which serves to bolster the umami quotient in a dish, without interference from other flavors. I think of this as one of the earliest forms of molecular gastronomy. MSG has quite a reputation as an unhealthy additive in Chinese and other Asian cuisines, but despite decades of research, no conclusive evidence links MSG to high blood pressure or other health anomalies. That said, like anything else, it is probably wise to consume it in moderation.
Umami, however, can come from so many other sources, mostly from protein rich foods, or, better yet, ingredients that have degraded proteins. Miso (pictured) is a very umami rich ingredient, as are the different fish sauces, and fermented tofu. And, of course, umami is likely to big reason behind using broths and stocks in Western style cooking, as there, the central focus of protein comes from animal meat and bones.