The key ingredient is Taurine
I've read it was originally extracted from bull hearts, but is now synthetic of course.
This link has info on Taurine as well as the quantity's in several popular energy drinks.
http://en.wikipedia.org/wiki/Taurine
The other active ingredients are Caffeine, Sugar, and sometimes Guarana(a stimulant related to caffeine)
The [censored] gives me a headache