I'm going to go out on a limb here and say lemons! hahaha ;)
In all seriousness, lemons are a fabulous and natural way to lighten your skin that isn't too drying or harmful to the skin. You can use lemons to lighten your skin as a whole, or you could use it as a spot treatment for any sunspots. Just remember that lemon also makes your skin quite susceptible to bad-for-you UV rays, so always make sure to slather on the sunscreen!
If you're looking for a more daily type of treatmentI know a few girls who have made this recipe for their morning cleanser:
Combine equal parts water, fresh lemon juice, honey, and aloe and use it the way you would use any facial cleanser - wash your face with it using a circular motion, let it sink in for a few minutes, rinse off, pat dry, moisturize. It keeps for a few days and is super cheap :-)
Hope that helps!