Photo: Jorge Barrios [wikipedia]
Neatorama reader Jon Jason sent in this post from Scienceray titled Top 5 Animals with the Most Stunning Eyes - I was most fascinated with the cat with two different eye colors. It turns out, heterochromia (difference in coloration) of the eye is caused by the difference in concentration and distribution of the pigment melanin.
In cats, a form of heterochromia results in odd-eyed cats [wiki] - they are most commonly found in white cats and is due to the white spotting gene. In these cats, melanin granules are prevented from reaching one eye during development, resulting in a cat with one blue eye and one green, yellow, or brown eye.