Glutamate is an amino acid responsible for the fifth primary taste grouping, savory. (the other four are bitter, salty, sour and sweet). The savory taste, also known as umami* (Japanese for delicious or savory), is the taste associated with things like tomatoes, seaweed, and beef.

Glutamate, or glutamic acid, was first identified in 1908 by Kikaune Ikeda. Glutamates are tremendous taste enhancers, hence the general (but certainly not universal) human predisposition to enjoy things like meat. Glutamate is also responsible for the tremendous power of MSG, which, of course, combines a glutamate with sodium to pack a double-whammy of savory and salt.

According to the glutamic acid writeup, it has a similar molecular structure to aspartic acid, one of the components of aspartame, or NutraSweet.

Sources:
http://chemistry.about.com/science/chemistry/library/weekly/aa062600a.htm
http://ificinfo.health.org/review/ir-msg.htm

* cool wu by sensei