There are at least 63 plant species known to produce caffeine(1). The most important harvested plants are coffee, tea, cacao, maté, kola (the original caffeine ingredient in carbonated drinks) and guarana (common in health supplements).
In tea and maté, the leaves are harvested; for coffee, cacao, kola and guarana, the fruit and seeds. The highest concentrations of caffeine tend to occur in tea and guarana, but all species can vary widely in caffeine content, depending on variety, climate, and cultivation (Graham, 2009), and whether the plant is male or female. Some reported concentrations:
Plant_______ Scientific Name_____ Caffeine content___Source
Tea________Camellia sinensis_____2.7-4.1%__________Kaplan et al, 1974
Guarana____Paullinia cupana______1.6-4.3%__________Baumann et al, 1995
Kola_______Cola acuminate_______1-2.2%___________Somorin, 1973
___________& Cola nitida
Coffee______Coffea arabica_______0.8-1.8%__________Kaplan et al, 1974
Maté_______Yerba mate__________0.8-1.7%__________Dellacassa et al, 2007
Cacao______Theobroma cacao____0.07-1.7%_________Asamoa_and Wurziger, 1976
On top of this natural variation, preparation of a serving can greatly vary the strength of the caffeine dose. A prepared cup of regular coffee (not counting decaf) can vary five-fold in caffeine concentration among brands and methods (McCusker et al, 2003). The same study found, after purchasing the same beverage from the same coffee shop for six days in a row, that the dose among those six cups varied up to a factor of two, from 259-564mg per cup.