  The ratio of shear viscosity to volume density of entropy can be used to
characterize how close a given fluid is to being perfect. Using string theory
methods, we show that this ratio is equal to a universal value of $\hbar/4\pi
k_B$ for a large class of strongly interacting quantum field theories whose
dual description involves black holes in anti--de Sitter space. We provide
evidence that this value may serve as a lower bound for a wide class of
systems, thus suggesting that black hole horizons are dual to the most ideal
fluids.
