Ripped out of context from another site:
"Glutamate and Glutamic acid are naturally occurring amino acids present in a large range of foods. It is free Glutamate that provides the unami taste.
They are most concentrated in sharp cheeses (like Parmesan), yeast and yeast extracts, anchovies, fermented fish sauce (and hence Worcestershire sauce), fermented bean products (hence soy sauce), sea-weed/vegetables and savory mushrooms (like Shiitaki). Tomatoes also contain reasonable amounts, which in the quantities of a tomato sauce, have a strong unami taste."