Perhaps you've seen someone with different color eyes, or skin with different color pigmentation; this is most likely a condition called Heterochromia. The term derives from the Ancient Greek words "heteros" which translates to different and "chromas" which translates to color. Heterochromia is simply a surplus or lack of melanin in one or more areas of the body. It isn't restricted to simply your eyeballs; it can occur in your hair, skin, nails and even your teeth.
Heterochromia can be caused by disease, or injury, but is most commonly due to genetics - usually acquired shortly after birth. While it's a visually arresting thing to see, the condition isn't common; in fact, roughly eleven out of every thousand people will experience this mutation.
Some congenital syndromes that may also be referred to as Heterochromia include: Waarenburg Syndrome, which is also genetically acquired and causes possible loss of hearing and changes in coloring of the subjects skin, hair or eyes; Piebaldism causes patches of different colored skin throughout the body and hair; and Incontinentia Pigment or 'Block-Sulzberger Syndrome' affects the pigment of hair, teeth, nails, and central nervous system. The iris may also be darker in the affected eye. That's just to name a few - there are many more variations.
Heterochromia isn't a recent occurrence. Emperor Anastasius, an Eastern Roman Emperor from 491 to 518, was one of the first known cases with this condition. Anastasius had one black eye and one blue, and for this reason was nicknamed Dicorous, which means "two- pupil" in Greek. Alexander The Great was also allegedly similarly affected.
The condition isn't even confined to humans; many pets have it as well. Siberian Huskies, for example, almost always have hetrochromia. It seems to be because of the way huskies were originally bred, and the other breed allowing blue eyes. The standard eye color for Alaskan malamutes, a different breed of dog, is brown; Heterochromia wouldn't occur in any case.
Another explanation could be the Tyndall effect, also known as Tyndall scattering; this occurs when light is scattered by particles in a colloid or else particles in a very fine suspension. The power of the scattered light relies on the fourth power of the frequency - blue light is simply being scattered much more drastically than red light.
With huskies, the way that the Tyndall effect causes Heterochromia to occur is by a translucent layer in the iris. Brown and black irises are the same layer but with more or less melanin in it. Melanin absorbs light like a sponge and reflects darkness; a complete lack of melanin means that the layer would be translucent, thus the light coming through it would be scattered randomly. Another example of this that we see every day would be the sky itself or the ocean - making them appear blue when there's actually no color at all.
While Heterochromia may not be good enough to get someone into Xaviers School For Gifted Youngsters, it's still pretty amazing.