I've found that Neutrogena 3-in-1 Hydrating Acne Treatment
makes the spots left behind from acne fade more quickly. It's supposedly the cheapest product that contains salicylic acid that has the right pH to let the SA actually work.
Post-zit hyperpigmentation is tiny amounts of iron left over from blood cells. It takes your body a little while to break it down. Chemical exfoliants like SA can speed this process up.