Never heard of it either. Here is a quick cut and paste and also links to some info:
In the years since its discovery, grapefruit seed extract has been tested and validated by the FDA, USDA, the Pasteur Institute in France, and numerous physicians. The active ingredient of grapefruit seed extract is non-toxic and is synthesized from the seed and pulp of certified organically grown grapefruit. The process converts the grapefruit bioflavonoids (polyphenolics) into an extremely potent compound that is being used to kill strep, staph, salmonella, E. Coli, candida, herpes, influenza, parasites, fungi, and more.
Did a little more reading. I found this interesting, it's on the first link above:
Researchers at the University of Sao Paulo in Brazil found the substance to be 100% effective in skin disinfection when used as a pre-surgical preparation compared to a 72% effectiveness rate for alcohol.