Yet another Brazilian plant associated with love potions, guarana is a caffeine-bearing jungle vine. Its bark is the interesting bit, having three times the caffeine of coffee. Caffeine is, in fact, its only active ingredient, meaning that contrary to its reputation it is not directly an aphrodisiac.
Now, caffeine in moderation has its uses. Guarana is therefore useful for fending off drowsiness or mental exhaustion, suppressing one's appetite (for food, of course) and even driving away a hangover. Do be careful with it, though, as caffeine in excessive amounts causes problems.