Stevia - approved for use as a food supplement in the US in 1995, I believe.
It's a south American herb which is estimated to be 30 times sweeter than
cane sugar.  In crystalline form, it is approx. 300 times sweeter than
sugar.  It has apparently been used by the Guarani Indians of Paraguay for
centuries.   One website is
http://www.healthfree.com/stevlif.htm

Legal or not?

Chris