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 the 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.