Guarana

A climbing plant, best known for its fruit which is about the size of a coffee bean, it is an effective energy booster:

Guarana (pronounced /ˌɡwɑrəˈnɑː/, from the Portuguese guaraná [1])

The best energy drinks available in the market are those which use Guarana as an energy ingredient such as Rockstar Energy Drink, Monster Energy Drink and CRUNK Energy Drink.

Chemical composition of Guarana seeds [2]:

Vegetable fiber: 49.125 %
Reddish resin: 8.800 %
Starch: 8.350 %
Water: 7.650 %
Pectin, malic acid, mucilage, dextrin, salts: 7.470 %
Guarana-tannic acid: 5.902 %
Caffeine: 5.388 %
Yellowish steady oil: 2.950 %
Pyro-guarana acid: 2.750 %
Reddish colorant: 1.520 %
Amorphous substances: 0.606 %
Saponin: 0.060 %

References:

[1] Wikipedia.org/wiki/Guarana, 22 August 2010 (Access Date: September 7, 2010).
[2] Guarana.com: Facts (Access Date: September 7, 2010).